We can't get a straight answer on the exact jailbroken status of iPhone OS 4, but MuscleNerd of the iPhone Dev Team (and recent iPad hacking fame) is sure hinting heavily. He's got Veency (a VNC server) and Cydia running on iPhone OS 4 on video, something he claims "you'll only find on JB 4.0." Of course, he also has the dev tools at his disposal and all the bugs and loopholes of an early beta, so we can't be exactly sure what's going on. There's also not much point to releasing an OS 4 beta jailbreak to the public, and MuscleNerd himself recommends not bothering with the betas, as they're "very buggy." Still, it's fun to see
