Same problem using a Sierrra Wireless USB dongle with SprintPCS (USA).  Quoth the Syslog, "

May 18 14:24:45 dan-laptop kernel: [ 1794.744259] usb 4-1: new full speed USB device using ohci_hcd and address 4                                             
May 18 14:24:45 dan-laptop kernel: [ 1794.964994] usb 4-1: configuration #1 chosen from 1 choice                                                              
May 18 14:24:45 dan-laptop kernel: [ 1794.990336] usb-storage: probe of 4-1:1.0 failed with error -5                                                          
May 18 14:24:45 dan-laptop kernel: [ 1794.994658] usb 4-1: USB disconnect, address 4                                                                          
May 18 14:24:47 dan-laptop kernel: [ 1796.540260] usb 4-1: new full speed USB device using ohci_hcd and address 5                                             
May 18 14:24:47 dan-laptop kernel: [ 1796.755469] usb 4-1: configuration #1 chosen from 1 choice                                                              
May 18 14:24:47 dan-laptop kernel: [ 1796.758431] sierra 4-1:1.0: Sierra USB modem converter detected                                                         
May 18 14:24:47 dan-laptop kernel: [ 1796.761473] usb 4-1: Sierra USB modem converter now attached to ttyUSB0                                                 
May 18 14:24:47 dan-laptop kernel: [ 1796.761552] usb 4-1: Sierra USB modem converter now attached to ttyUSB1                                                 
May 18 14:24:47 dan-laptop kernel: [ 1796.761622] usb 4-1: Sierra USB modem converter now attached to ttyUSB2                                                 
May 18 14:24:47 dan-laptop kernel: [ 1796.770653] scsi8 : SCSI emulation for USB Mass Storage devices                                                         
May 18 14:24:47 dan-laptop kernel: [ 1796.771878] usb-storage: device found at 5                                                                              
May 18 14:24:47 dan-laptop kernel: [ 1796.771882] usb-storage: waiting for device to settle before scanning                                                   
May 18 14:24:48 dan-laptop nm-system-settings:    SCPlugin-Ifupdown: device added (udi: /org/freedesktop/Hal/devices/usb_device_1199_23_noserial_if0_serial_usb_0, iface: (null)): iface not found
May 18 14:24:48 dan-laptop NetworkManager: <info>  (ttyUSB0): found serial port (udev:CDMA  hal:CDMA)
May 18 14:24:48 dan-laptop NetworkManager: <info>  (ttyUSB0): new Modem device (driver: 'sierra')
May 18 14:24:48 dan-laptop NetworkManager: <info>  (ttyUSB0): exported as /org/freedesktop/Hal/devices/usb_device_1199_23_noserial_if0_serial_usb_0
May 18 14:24:52 dan-laptop NetworkManager: <info>  (ttyUSB0): device state change: 1 -> 2
May 18 14:24:52 dan-laptop NetworkManager: <info>  (ttyUSB0): deactivating device (reason: 2).
May 18 14:24:52 dan-laptop NetworkManager: <info>  Policy set 'Gemini_Techies' (wlan0) as default for routing and DNS.
May 18 14:24:52 dan-laptop NetworkManager: nm_system_device_flush_ip4_routes_with_iface: assertion `iface_idx >= 0' failed
May 18 14:24:52 dan-laptop NetworkManager: nm_system_device_flush_ip4_addresses_with_iface: assertion `iface_idx >= 0' failed
May 18 14:24:52 dan-laptop NetworkManager: <info>  (ttyUSB0): device state change: 2 -> 3
May 18 14:24:52 dan-laptop kernel: [ 1801.770434] usb-storage: device scan complete
May 18 14:24:52 dan-laptop kernel: [ 1801.774379] scsi 8:0:0:0: Direct-Access     Sierra   Wireless Storage 2.31 PQ: 0 ANSI: 2
May 18 14:24:52 dan-laptop kernel: [ 1801.789505] sd 8:0:0:0: [sdc] Attached SCSI removable disk
May 18 14:24:52 dan-laptop kernel: [ 1801.789638] sd 8:0:0:0: Attached scsi generic sg3 type 0
May 18 14:25:01 dan-laptop NetworkManager: <info>  Activation (ttyUSB0) starting connection 'Sprint PCS'
May 18 14:25:01 dan-laptop NetworkManager: <info>  (ttyUSB0): device state change: 3 -> 4
May 18 14:25:01 dan-laptop NetworkManager: <info>  Activation (ttyUSB0) Stage 1 of 5 (Device Prepare) scheduled...
May 18 14:25:01 dan-laptop NetworkManager: <info>  Activation (ttyUSB0) Stage 1 of 5 (Device Prepare) started...
May 18 14:25:01 dan-laptop NetworkManager: <debug> [1242674701.151750] nm_serial_device_open(): (ttyUSB0) opening device...
May 18 14:25:01 dan-laptop NetworkManager: Invalid baudrate '32298490'
May 18 14:25:01 dan-laptop NetworkManager: Invalid bits (0). Valid values are 5, 6, 7, 8.
May 18 14:25:01 dan-laptop NetworkManager: Invalid parity (
May 18 14:25:01 dan-laptop NetworkManager: Invalid stop bits (0). Valid values are 1 and 2)
May 18 14:25:01 dan-laptop NetworkManager: <info>  Activation (ttyUSB0) Stage 1 of 5 (Device Prepare) complete.
May 18 14:25:22 dan-laptop NetworkManager: <info>  (ttyUSB0): powering up...
May 18 14:26:47 dan-laptop NetworkManager: <WARN>  dial_done(): No carrier
May 18 14:26:47 dan-laptop NetworkManager: <info>  (ttyUSB0): device state change: 4 -> 9
May 18 14:26:47 dan-laptop NetworkManager: <debug> [1242674807.981168] nm_serial_device_close(): Closing device 'ttyUSB0'
May 18 14:26:47 dan-laptop NetworkManager: <info>  Marking connection 'Sprint PCS' invalid.
May 18 14:26:47 dan-laptop NetworkManager: <info>  Activation (ttyUSB0) failed.
May 18 14:26:47 dan-laptop NetworkManager: <info>  (ttyUSB0): device state change: 9 -> 3
May 18 14:26:47 dan-laptop NetworkManager: <info>  (ttyUSB0): deactivating device (reason: 0).
May 18 14:26:47 dan-laptop NetworkManager: <info>  Policy set 'Gemini_Techies' (wlan0) as default for routing and DNS.
May 18 14:26:47 dan-laptop NetworkManager: nm_system_device_flush_ip4_routes_with_iface: assertion `iface_idx >= 0' failed
May 18 14:26:47 dan-laptop NetworkManager: nm_system_device_flush_ip4_addresses_with_iface: assertion `iface_idx >= 0' failed

"

dan@dan-laptop:/var/log$ lsb_release -rd
Description:    Ubuntu 9.04
Release:        9.04


I'm guessing some pppd options got hosed somewhere; where can I change these?