iPhone OS 4.0 has only been available to developers for a matter of hours, but already there's been a jailbreak. MuscleNerd, of the infamous iPhone Dev Team, has tweeted a video showing Cydia and Veency running on his iPhone.
Video demo after the cut
So far there are no details and no guidelines as to how to do it yourself, but there's plenty of time for it to be polished before anyone is likely to actually need it. We're presuming the same system will probably work for the iPad, too, when that gets OS 4.0 this coming fall.
MuscleNerd also has a little advice for anyone considering putting an early developer release of iPhone OS 4.0 on their iPhone: don't bother. Turns out, it's still "very" buggy - hence why it's not set to actually release to general users until Summer 2010.