Holy crap, that didn't take long.
Less than 24 hours after iPhone OS 4.0's coming-out party, Dev-Team member "MuscleNerd" posted the following on his Twitter account:
Something you'll only find on JB 4.0: VNC 🙂 http://is.gd/blzeg (BTW don't bother with betas, they're *very* buggy!)
The link points to a Youtube video (seen above) showing a jailbroken version of OS 4.0 running Cydia and Veency.
How the HELL did they pwn it this quickly? One theory: the Dev-Team utilized a previously-unexploited loophole found while jailbreaking OS 3.1.3, one that was carried over into 4.0. It's safe to say, though, that Apple will have pinned this particular bug to the wall long before the new OS actually hits retail channels this fall.