I have a Huawei Gobi 3000 device that has a GSM firmware loaded and provides 3 ttyUSB interfaces via qcserial. 
I can connect and use the device via pppd without a problem but modemmanager cannot detect the device.
Running on Debian Sid with modemmanager version 0.5-1
The serial device that pppd works with is ttyUSB1 and here is the output I get when running modem-manager --debug

modem-manager[6245]: <info>  [1316615687.378757] [main.c:167] main(): ModemManager (version 0.5) starting...
modem-manager[6245]: <info>  [1316615687.383918] [mm-manager.c:120] load_plugin(): Loaded plugin Option High-Speed
modem-manager[6245]: <info>  [1316615687.384393] [mm-manager.c:120] load_plugin(): Loaded plugin Gobi
modem-manager[6245]: <info>  [1316615687.384854] [mm-manager.c:120] load_plugin(): Loaded plugin Option
modem-manager[6245]: <info>  [1316615687.385291] [mm-manager.c:120] load_plugin(): Loaded plugin MotoC
modem-manager[6245]: <info>  [1316615687.385729] [mm-manager.c:120] load_plugin(): Loaded plugin Linktop
modem-manager[6245]: <info>  [1316615687.386162] [mm-manager.c:120] load_plugin(): Loaded plugin Longcheer
modem-manager[6245]: <info>  [1316615687.386582] [mm-manager.c:120] load_plugin(): Loaded plugin X22X
modem-manager[6245]: <info>  [1316615687.387050] [mm-manager.c:120] load_plugin(): Loaded plugin Novatel
modem-manager[6245]: <info>  [1316615687.387468] [mm-manager.c:120] load_plugin(): Loaded plugin Generic
modem-manager[6245]: <info>  [1316615687.387904] [mm-manager.c:120] load_plugin(): Loaded plugin Samsung
modem-manager[6245]: <info>  [1316615687.388325] [mm-manager.c:120] load_plugin(): Loaded plugin Nokia
modem-manager[6245]: <info>  [1316615687.388748] [mm-manager.c:120] load_plugin(): Loaded plugin SimTech
modem-manager[6245]: <info>  [1316615687.389193] [mm-manager.c:120] load_plugin(): Loaded plugin Ericsson MBM
modem-manager[6245]: <info>  [1316615687.389637] [mm-manager.c:120] load_plugin(): Loaded plugin ZTE
modem-manager[6245]: <info>  [1316615687.390064] [mm-manager.c:120] load_plugin(): Loaded plugin Wavecom
modem-manager[6245]: <info>  [1316615687.390492] [mm-manager.c:120] load_plugin(): Loaded plugin Sierra
modem-manager[6245]: <info>  [1316615687.390977] [mm-manager.c:120] load_plugin(): Loaded plugin Huawei
modem-manager[6245]: <info>  [1316615687.391404] [mm-manager.c:120] load_plugin(): Loaded plugin AnyData
modem-manager[6245]: <debug> [1316615687.397584] [mm-manager.c:809] device_added(): (tty/ttyS0): port's parent platform driver is not whitelisted
modem-manager[6245]: <debug> [1316615687.397994] [mm-manager.c:809] device_added(): (tty/ttyS1): port's parent platform driver is not whitelisted
modem-manager[6245]: <debug> [1316615687.398337] [mm-manager.c:809] device_added(): (tty/ttyS2): port's parent platform driver is not whitelisted
modem-manager[6245]: <debug> [1316615687.398667] [mm-manager.c:809] device_added(): (tty/ttyS3): port's parent platform driver is not whitelisted
modem-manager[6245]: <info>  [1316615687.404036] [mm-serial-port.c:741] mm_serial_port_open(): (ttyUSB1) opening serial port...
modem-manager[6245]: <debug> [1316615687.404123] [mm-serial-port.c:810] mm_serial_port_open(): (ttyUSB1) device open count is 1 (open)
modem-manager[6245]: <debug> [1316615687.404141] [mm-plugin-base.c:850] try_open(): (ttyUSB1): probe requested by plugin 'Gobi'
modem-manager[6245]: <debug> [1316615687.404225] [mm-at-serial-port.c:298] debug_log(): (ttyUSB1): <-- '<CR><LF>+PACSP0<CR><LF>'
modem-manager[6245]: <debug> [1316615687.504556] [mm-at-serial-port.c:298] debug_log(): (ttyUSB1): --> 'AT+GCAP<CR>'
modem-manager[6245]: <debug> [1316615688.204741] [mm-at-serial-port.c:298] debug_log(): (ttyUSB1): <-- 'AT+GCAP'
modem-manager[6245]: <debug> [1316615688.206753] [mm-at-serial-port.c:298] debug_log(): (ttyUSB1): <-- '<CR>'
modem-manager[6245]: <debug> [1316615688.207998] [mm-at-serial-port.c:298] debug_log(): (ttyUSB1): <-- '<CR><LF>+GCAP: +CGSM,+DS,+ES<CR><LF><CR><LF>OK<CR><LF>'
modem-manager[6245]: <debug> [1316615688.208129] [mm-serial-port.c:844] mm_serial_port_close(): (ttyUSB1) device open count is 0 (close)
modem-manager[6245]: <info>  [1316615688.208149] [mm-serial-port.c:859] mm_serial_port_close(): (ttyUSB1) closing serial port...
modem-manager[6245]: <info>  [1316615688.208691] [mm-serial-port.c:880] mm_serial_port_close(): (ttyUSB1) serial port closed
modem-manager[6245]: <debug> [1316615688.215410] [mm-manager.c:491] try_supports_port(): (Huawei): (ttyUSB1) deferring support check
modem-manager[6245]: <debug> [1316615691.218642] [mm-manager.c:457] supports_defer_timeout(): (ttyUSB1): re-checking support...
modem-manager[6245]: <debug> [1316615691.219568] [mm-manager.c:491] try_supports_port(): (Huawei): (ttyUSB1) deferring support check



after that I only get the re-checking support and deffering support check messages.

lsusb -v output:

Bus 001 Device 019: ID 12d1:14f1 Huawei Technologies Co., Ltd. 
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.00
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0        64
  idVendor           0x12d1 Huawei Technologies Co., Ltd.
  idProduct          0x14f1 
  bcdDevice            0.02
  iManufacturer           3 HUAWEI Incorporated
  iProduct                2 Huawei EM680 w/Gobi Technology
  iSerial                 0 
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength          157
    bNumInterfaces          4
    bConfigurationValue     1
    iConfiguration          1 Qualcomm Configuration
    bmAttributes         0xe0
      Self Powered
      Remote Wakeup
    MaxPower              500mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           3
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass    255 Vendor Specific Subclass
      bInterfaceProtocol    255 Vendor Specific Protocol
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0040  1x 64 bytes
        bInterval               5
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x82  EP 2 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval              32
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x01  EP 1 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval              32
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        1
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass    255 Vendor Specific Subclass
      bInterfaceProtocol    255 Vendor Specific Protocol
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x83  EP 3 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval              32
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x02  EP 2 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval              32
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        2
      bAlternateSetting       0
      bNumEndpoints           3
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass    255 Vendor Specific Subclass
      bInterfaceProtocol    255 Vendor Specific Protocol
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x84  EP 4 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0040  1x 64 bytes
        bInterval               5
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x85  EP 5 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval              32
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x03  EP 3 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval              32
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        3
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass    255 Vendor Specific Subclass
      bInterfaceProtocol    255 Vendor Specific Protocol
      iInterface              0 
      ** UNRECOGNIZED:  05 24 00 10 01
      ** UNRECOGNIZED:  05 24 15 00 01
      ** UNRECOGNIZED:  05 24 06 03 03
      ** UNRECOGNIZED:  15 24 12 20 01 98 b0 6a 49 b0 9e 48 96 94 46 d9 9a 28 ca 4e 5d
      ** UNRECOGNIZED:  06 24 13 00 01 20
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x86  EP 6 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval              32
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x04  EP 4 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval              32
Device Qualifier (for other device speed):
  bLength                10
  bDescriptorType         6
  bcdUSB               2.00
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0        64
  bNumConfigurations      1
Device Status:     0x0000
  (Bus Powered)