Finally some progress, I can stop the act of inserting the AT&T Quicksilver in the USB port from locking the system,
and I may have just isolated what is really causing the lockup under Ubuntu 9.10.

Go ahead download, build and install the Ozerocdoff package from http://pharscape.org/Quicksilver.html

To stop the lockup from happening before you insert the Quicksilver, fire up the Synaptic Package Manager
Find the modemmanager application and mark it for complete removal, then have Synaptic apply the change

The modem-manager application is grabbing the ttyHS* ports, specifically ttyHS2 which is causing the lockup.

Nov 19 19:21:07 dragon modem-manager: Added modem /sys/devices/pci0000:00/0000:00:1e.0/0000:02:07.2/usb1/1-5
Nov 19 19:21:07 dragon modem-manager: data_available (ttyHS2): response buffer filled before repsonse received
Nov 19 19:21:07 dragon modem-manager: data_available (ttyHS2): response buffer filled before repsonse received
Nov 19 19:21:07 dragon modem-manager: data_available (ttyHS2): response buffer filled before repsonse received
Nov 19 19:21:07 dragon modem-manager: data_available (ttyHS2): response buffer filled before repsonse received

After you remove modem-manager this is what happens in /var/log/messages when you insert the AT&T Quicksilver

Nov 19 19:57:44 dragon kernel: [ 2093.880060] usb 1-5: new high speed USB device using ehci_hcd and address 8
Nov 19 19:57:44 dragon kernel: [ 2094.014102] usb 1-5: configuration #1 chosen from 1 choice
Nov 19 19:57:44 dragon kernel: [ 2094.016719] scsi3 : SCSI emulation for USB Mass Storage devices
Nov 19 19:57:46 dragon kernel: [ 2095.180193] usb 1-5: USB disconnect, address 8
Nov 19 19:57:46 dragon kernel: [ 2095.432102] usb 1-5: new high speed USB device using ehci_hcd and address 9
Nov 19 19:57:46 dragon kernel: [ 2095.570090] usb 1-5: configuration #1 chosen from 1 choice
Nov 19 19:57:47 dragon kernel: [ 2096.244377] hso: /build/buildd/linux-2.6.31/drivers/net/usb/hso.c: 1.2 Option Wireless
Nov 19 19:57:47 dragon kernel: [ 2096.245439] hso0: Disabled Privacy Extensions
Nov 19 19:57:47 dragon kernel: [ 2096.246866] usbcore: registered new interface driver hso
Nov 19 19:57:52 dragon kernel: [ 2101.596257] Modules linked in: hso binfmt_misc usblp snd_intel8x0 snd_ac97_codec pcmcia hostap_pci hostap lib80211 yenta_socket ac97_bus snd_pcm_oss orinoco_pci rsrc_nonstatic joydev snd_mixer_oss orinoco pcmcia_core shpchp snd_pcm snd_seq_dummy snd_seq_oss nls_iso8859_1 snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq nls_cp437 psmouse serio_raw ppdev snd_timer vfat lp iptable_filter snd_seq_device fat sony_laptop ip_tables parport snd soundcore snd_page_alloc x_tables usbhid usb_storage e100 mii ohci1394 ieee1394 radeon ttm drm i2c_algo_bit intel_agp agpgart
Nov 19 19:57:52 dragon kernel: [ 2101.596368] 
Nov 19 19:57:52 dragon kernel: [ 2101.596380] Pid: 537, comm: modem-manager Tainted: G        W  (2.6.31-14-generic #48-Ubuntu) PCG-V505BX(UC)      
Nov 19 19:57:52 dragon kernel: [ 2101.596391] EIP: 0073:[<00383b07>] EFLAGS: 00010246 CPU: 0
Nov 19 19:57:52 dragon kernel: [ 2101.596405] EIP is at 0x383b07
Nov 19 19:57:52 dragon kernel: [ 2101.596413] EAX: 00000000 EBX: 0044eff4 ECX: 00000008 EDX: 00000000
Nov 19 19:57:52 dragon kernel: [ 2101.596422] ESI: 00000000 EDI: 0816a2fc EBP: bfb93828 ESP: bfb937f4
Nov 19 19:57:52 dragon kernel: [ 2101.596431]  DS: 007b ES: 007b FS: 0000 GS: 0033 SS: 007b
Nov 19 19:57:52 dragon kernel: [ 2101.596440] CR0: 8005003b CR2: 0805c660 CR3: 31911000 CR4: 000006d0
Nov 19 19:57:52 dragon kernel: [ 2101.596449] DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
Nov 19 19:57:52 dragon kernel: [ 2101.596458] DR6: ffff0ff0 DR7: 00000400
Nov 19 19:57:52 dragon kernel: [ 2101.596465] Call Trace:

So there are obviously still problems but at least the system does not lock up

$ ls -l /dev/ttyHS*
crw-rw---- 1 root dialout 250, 0 2009-11-19 19:57 /dev/ttyHS0
crw-rw---- 1 root dialout 250, 1 2009-11-19 19:57 /dev/ttyHS1
crw-rw---- 1 root dialout 250, 2 2009-11-19 19:57 /dev/ttyHS2
crw-rw---- 1 root dialout 250, 3 2009-11-19 19:57 /dev/ttyHS3
crw-rw---- 1 root dialout 250, 4 2009-11-19 19:57 /dev/ttyHS4

The devices are now all there

$ lsusb
Bus 003 Device 003: ID 054c:0069 Sony Corp. Memorystick MSC-U03 Reader
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 009: ID 0af0:d033 Option 
Bus 001 Device 007: ID 0461:4d15 Primax Electronics, Ltd Dell Optical Mouse
Bus 001 Device 006: ID 413c:2003 Dell Computer Corp. Keyboard
Bus 001 Device 005: ID 05e3:0606 Genesys Logic, Inc. USB 2.0 Hub / D-Link DUB-H4 USB 2.0 Hub
Bus 001 Device 003: ID 0409:0058 NEC Corp. HighSpeed Hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub

Debian's modem manager application needs to die die die.