I have the same or a similar problem with a brand new Verbatim Store'n'go 1 GB USB stick. When writing a lot of files (say 4000 files of 80MB) to the stick, I got an error message and it got unmounted. After that, all data and even the partition table was lost! Because of a possible manufacturing error I returned the stick and had it replaced with another one (same model). The first thing I did on the new one was
dd if=/dev/zero of=/dev/sdb 
then I created a FAT32 partition. I still get lots of write errors on that one, but at least the partition table stays intact (maybe due to the "dd..."?).

I have the problem on Edgy (all updates installed) and tested with a live CD of Feisty Beta as well. Same error on both (on a IBM Thinkpad R52). Here's the log on Feisty Beta:

Mar 31 19:54:03 ubuntu kernel: [    9.752000] scsi 2:0:0:0: Direct-Access     VBTM     Store'n'go Pro   6.50 PQ: 0 ANSI: 0 CCS
Mar 31 19:54:03 ubuntu kernel: [    9.752000] SCSI device sdb: 1989631 512-byte hdwr sectors (1019 MB)
Mar 31 19:54:03 ubuntu kernel: [    9.752000] sdb: Write Protect is off
Mar 31 19:54:03 ubuntu kernel: [    9.752000] SCSI device sdb: 1989631 512-byte hdwr sectors (1019 MB)
Mar 31 19:54:03 ubuntu kernel: [    9.756000] sdb: Write Protect is off
Mar 31 19:54:03 ubuntu kernel: [    9.756000]  sdb: sdb1
Mar 31 19:54:03 ubuntu kernel: [    9.756000] sd 2:0:0:0: Attached scsi removable disk sdb
Mar 31 19:54:03 ubuntu kernel: [    9.756000] sd 2:0:0:0: Attached scsi generic sg2 type 0

Here's where the error occurs (while copying files to the stick):

Mar 31 19:58:58 ubuntu kernel: [  497.428000] usb 5-3.2: reset high speed USB device using ehci_hcd and address 3
Mar 31 19:59:17 ubuntu kernel: [  515.852000] usb 5-3.2: reset high speed USB device using ehci_hcd and address 3
Mar 31 19:59:35 ubuntu kernel: [  534.276000] usb 5-3.2: reset high speed USB device using ehci_hcd and address 3
Mar 31 19:59:45 ubuntu kernel: [  544.592000] usb 5-3.2: reset high speed USB device using ehci_hcd and address 3
Mar 31 19:59:56 ubuntu kernel: [  554.836000] sd 2:0:0:0: scsi: Device offlined - not ready after error recovery
Mar 31 19:59:56 ubuntu kernel: [  554.836000] sd 2:0:0:0: SCSI error: return code = 0x00050000
Mar 31 19:59:56 ubuntu kernel: [  554.836000] end_request: I/O error, dev sdb, sector 1854531
Mar 31 19:59:56 ubuntu kernel: [  554.836000] lost page write due to I/O error on sdb1
Mar 31 19:59:56 ubuntu last message repeated 9 times
Mar 31 19:59:56 ubuntu kernel: [  554.836000] sd 2:0:0:0: SCSI error: return code = 0x00010000
Mar 31 19:59:56 ubuntu kernel: [  554.836000] end_request: I/O error, dev sdb, sector 1855891
Mar 31 19:59:56 ubuntu kernel: [  555.268000] usb 5-3.2: USB disconnect, address 3
Mar 31 19:59:56 ubuntu kernel: [  555.496000] usb 5-3.2: new high speed USB device using ehci_hcd and address 6

After that, the stick is not mounted anymore. The light on the stick is always on after the error occured (usually it's off or it's blinking when the stick is accessed).

lsusb -v of the stick:

Bus 005 Device 006: ID 08ec:0020 M-Systems Flash Disk Pioneers 
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.00
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0        64
  idVendor           0x08ec M-Systems Flash Disk Pioneers
  idProduct          0x0020 
  bcdDevice            2.00
  iManufacturer           1 Verbatim
  iProduct                2 Store'n'go Pro
  iSerial                 3 0D70E17112807909
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           32
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          0 
    bmAttributes         0x80
      (Bus Powered)
    MaxPower              180mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass         8 Mass Storage
      bInterfaceSubClass      6 SCSI
      bInterfaceProtocol     80 Bulk (Zip)
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      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               0
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)