Musclenerd recently showed iPhone OS 4.0 jailbroken. While he didn't show any method or speak of one, the shots showed 4.0 busted into.
Now, geohot just tweeted an image of iPhone OS 4.0 (notice the Game Center icon) with blackra!n and Cydia both there as well.
Looks like he's got blackra!n already working for 4.0! The jailbreaks are coming fast and furious lately...