What, seriously? The iPhone 4 isn't even OUT yet and some enterprising crackers have already managed to jailbreak Cupertino's latest OS. A hacker who goes by msft.guy has just posted PwnageTool bundles for the 3GS and iOS 4 GM release. These will allow you to jailbreak an iPhone 3GS running the new software.
Redmond Pie reports that the iPhone Dev Team plans to release a full jailbreak and unlock for iOS 4 at some point later this month. It will probably prove safer to wait for this jailbreak than the current option. Leave the tinkering to the big boys, until things have settled enough that you feel confident you won't end up destroying your phone.