hello. 
I have the same problem with the device with this id (19d2:0031) 
mine is a ZTE MF637 hsupa usb modem. 

the same modem works ok in lucid. but not in oneiric.
you are correct that since it already has the id:0031 that means modeswitch has already switched it to the modem mode. because it first identifies as a CDrom with id 19d2:2000

syslog output:
Jan  4 20:29:53 smiki-AOD257 kernel: [15886.676212] usb 1-4: new high speed USB device number 19 using ehci_hcd
Jan  4 20:29:54 smiki-AOD257 mtp-probe: checking bus 1, device 19: "/sys/devices/pci0000:00/0000:00:1d.7/usb1/1-4"
Jan  4 20:29:54 smiki-AOD257 kernel: [15886.826122] scsi21 : usb-storage 1-4:1.0
Jan  4 20:29:54 smiki-AOD257 mtp-probe: bus: 1, device: 19 was not an MTP device
Jan  4 20:29:54 smiki-AOD257 usb_modeswitch: switching 19d2:2000 (ZTE,Incorporated: ZTE WCDMA Technologies MSM)
Jan  4 20:29:56 smiki-AOD257 kernel: [15888.811051] usb 1-4: USB disconnect, device number 19
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.180185] usb 1-4: new high speed USB device number 20 using ehci_hcd
Jan  4 20:29:56 smiki-AOD257 mtp-probe: checking bus 1, device 20: "/sys/devices/pci0000:00/0000:00:1d.7/usb1/1-4"
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.329564] option 1-4:1.0: GSM modem (1-port) converter detected
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.329962] usb 1-4: GSM modem (1-port) converter now attached to ttyUSB0
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.330304] option 1-4:1.1: GSM modem (1-port) converter detected
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.330705] usb 1-4: GSM modem (1-port) converter now attached to ttyUSB1
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.331373] scsi22 : usb-storage 1-4:1.2
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.332585] option 1-4:1.3: GSM modem (1-port) converter detected
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.333078] usb 1-4: GSM modem (1-port) converter now attached to ttyUSB2
Jan  4 20:29:56 smiki-AOD257 mtp-probe: bus: 1, device: 20 was not an MTP device
Jan  4 20:29:56 smiki-AOD257 modem-manager[11327]: <info>  (ttyUSB0) opening serial port...
Jan  4 20:29:56 smiki-AOD257 modem-manager[11327]: <info>  (ttyUSB2) opening serial port...
Jan  4 20:29:56 smiki-AOD257 modem-manager[11327]: <info>  (ttyUSB1) opening serial port...
Jan  4 20:29:57 smiki-AOD257 kernel: [15890.337033] scsi 22:0:0:0: Direct-Access     ZTE      MMC Storage      2.31 PQ: 0 ANSI: 2
Jan  4 20:29:57 smiki-AOD257 usb_modeswitch: switched to 19d2:0031 (ZTE,Incorporated: ZTE Configuration)
Jan  4 20:29:57 smiki-AOD257 kernel: [15890.557525] sd 22:0:0:0: Attached scsi generic sg2 type 0
Jan  4 20:29:57 smiki-AOD257 kernel: [15890.565528] sd 22:0:0:0: [sdc] Attached SCSI removable disk
Jan  4 20:29:58 smiki-AOD257 usb_modeswitch[11343]: usb_modeswitch: adding device ID 19d2:0031 to driver option
Jan  4 20:30:11 smiki-AOD257 modem-manager[11327]: <info>  (ttyUSB2) closing serial port...
Jan  4 20:30:11 smiki-AOD257 modem-manager[11327]: <info>  (ttyUSB2) serial port closed
Jan  4 20:30:11 smiki-AOD257 modem-manager[11327]: <info>  (ttyUSB2) opening serial port...
Jan  4 20:30:16 smiki-AOD257 modem-manager[11327]: <info>  (ttyUSB1) closing serial port...
Jan  4 20:30:16 smiki-AOD257 modem-manager[11327]: <info>  (ttyUSB1) serial port closed
Jan  4 20:30:16 smiki-AOD257 modem-manager[11327]: <info>  (ZTE): GSM modem /sys/devices/pci0000:00/0000:00:1d.7/usb1/1-4 claimed port ttyUSB1
Jan  4 20:30:17 smiki-AOD257 modem-manager[11327]: <info>  (ttyUSB2) closing serial port...
Jan  4 20:30:17 smiki-AOD257 modem-manager[11327]: <info>  (ttyUSB2) serial port closed
Jan  4 20:30:29 smiki-AOD257 modem-manager[11327]: <info>  (ttyUSB0) closing serial port...
Jan  4 20:30:29 smiki-AOD257 modem-manager[11327]: <info>  (ttyUSB0) serial port closed
Jan  4 20:30:29 smiki-AOD257 modem-manager[11327]: <info>  (ttyUSB0) opening serial port...
Jan  4 20:30:32 smiki-AOD257 modem-manager[11327]: <info>  (ttyUSB0) closing serial port...
Jan  4 20:30:32 smiki-AOD257 modem-manager[11327]: <info>  (ttyUSB0) serial port closed
Jan  4 20:30:32 smiki-AOD257 modem-manager[11327]: <info>  (ZTE): GSM modem /sys/devices/pci0000:00/0000:00:1d.7/usb1/1-4 claimed port ttyUSB0 

kernlog output: 
Jan  4 20:29:53 smiki-AOD257 kernel: [15886.676212] usb 1-4: new high speed USB device number 19 using ehci_hcd
Jan  4 20:29:54 smiki-AOD257 kernel: [15886.826122] scsi21 : usb-storage 1-4:1.0
Jan  4 20:29:56 smiki-AOD257 kernel: [15888.811051] usb 1-4: USB disconnect, device number 19
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.180185] usb 1-4: new high speed USB device number 20 using ehci_hcd
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.329564] option 1-4:1.0: GSM modem (1-port) converter detected
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.329962] usb 1-4: GSM modem (1-port) converter now attached to ttyUSB0
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.330304] option 1-4:1.1: GSM modem (1-port) converter detected
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.330705] usb 1-4: GSM modem (1-port) converter now attached to ttyUSB1
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.331373] scsi22 : usb-storage 1-4:1.2
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.332585] option 1-4:1.3: GSM modem (1-port) converter detected
Jan  4 20:29:56 smiki-AOD257 kernel: [15889.333078] usb 1-4: GSM modem (1-port) converter now attached to ttyUSB2
Jan  4 20:29:57 smiki-AOD257 kernel: [15890.337033] scsi 22:0:0:0: Direct-Access     ZTE      MMC Storage      2.31 PQ: 0 ANSI: 2
Jan  4 20:29:57 smiki-AOD257 kernel: [15890.557525] sd 22:0:0:0: Attached scsi generic sg2 type 0
Jan  4 20:29:57 smiki-AOD257 kernel: [15890.565528] sd 22:0:0:0: [sdc] Attached SCSI removable disk