Hello,
I recently updated from Leopard 10.5 to Snow Leopard 10.6. I have an HP Scanjet 4850 that was working fine until the upgrade.

After updating all the software updates, and grabbing the latest HP drivers for 10.6, the scanner still won't work. It is detected when I check the System Profiler. Also, when I run the HP Scanner software, the device is seen. However, when attempting a scan, nothing happens and I'm stuck.

Any advice?

After any upgrade open System Preferences->Print & Fax and delete the printer using the - sign. Then click on the + sign and re-add your printer/scanner. This should help.

HP ScanJet 4850 is a scanner only.
Why would you think that would appear in Print & Fax pane?

See if your 4850 will appear in your Image Capture app ...
Then, do you see a scanner window open that will let you scan anything?