Have a read of this for information on how to capture a couple of files that could be of use if you wish to jailbreak your 3GS that is running 3.0.

Do NOT do this if you're running 3.1, which was put in beta last week, and do NOT upgrade to the 3.1 beta if you want to use ultrasn0w.

A few notes for you. When you download the 3.0 firmware from the link in the instructions, remove the ".zip" part of the filename. Do NOT expand the zip file. This did not seem to be mentioned in the instructions and I've never done a jailbreak before so I wasn't aware of this. I had expanded the zip file and ended up with a folder ending with a .ipsw extension, which isn't what the images were displaying I should have. After a quick google it was revealed all I needed to do was remove the ".zip" portion of the filename and I'd get the file with the correct icon.