This process and links to download the IPSW file for each device are outlined in this helpful article by Gizmodo.
That should force i Tunes to again see the device and let you to restore your data from the recent backup.
Apple's support forums have been filled with messages from people who also bricked their devices through the update.
For those of you who haven't yet updated your device to IOS 5, what precautions should you take? In i Tunes right-click on your device and choose the "Back up" option.
After a restoration job that took a few hours, my i Pod Touch was back in business and fully loaded with i OS 5.
The only drawback is that this type of restore typically leaves your icons disorganized and floating outside their original folders.