Gizmodo is reporting that the Gold Master beta release of iOS 4 has already been cracked. Seeded to developers yesterday, the next iPhone's software is apparently accommodating to jailbreakers.
The crack was done by using existing jailbreak software Pwnagetool, by msft.guy. On top of this the iPhone Dev Team is claiming to have an unlock of their own for iOS 4 final.
So, on launch day, or near, of iOS 4 you should be able to jailbreak your handset, or at the very least your iPhone 3GS.
Instructions for the current jailbreak are for the 3GS only. The 4 isn't out yet! D'uh. Get the deets here, but it's probably a good idea to wait. The final release of iOS4 isn't until the end of this month, and a lot can change. Apple may have a few tricks up their sleeve, yet.