Updated to iOS4 and need to jailbreak your 3GS? First Redsn0w, and now PwnageTool give you the power. Here's a simple how-to guide to get you started. The caveat: this isn't an official release, so there may be some bugs that could damage your phone or delete some data. So proceed with caution! [Redmond Pie]
Not that anyone ever had any doubts that the Dev Team would be able to work their magic on the 3GS, but this is quick: within a week of the phone's launch, the hackers say they've figured it out.