After the developers' release of iOS4 was cracked earlier this month, the same team has pulled their finger out and released PwnageTool 4.01 for iPhone 3GS, iPhone 3G and the second-gen iPod Touch.
You'll need to have jailbroken your 3GS previously to make PwnageTool 4.01 work, but if you know what you're doing then definitely do check it out. I shouldn't have to tell you the first rule in gadget-hacking ("safety first"), so godspeed. [Dev-Team Blog]
Image Credit: _Zand
UPDATE: I didn't see Rosa's post on PwnageTool 4.0 two days ago, however the Dev Team's 4.01 update now supports iPhone 3GS, if you were wondering what the difference was.