They don't call him the wonder-boy for no reason. First! Geohot managed to hardware-unlock the iPhone EDGE as soon as it came out in 2007 and from there it all started. Until now, Geohot has managed to Jailbreak nearly every Apple device; on every OS, including the iPad.
And keeping up with this tradition, he has now even managed to Jailbreak the iPhone 4.0 Firmware using Blackra1n. The Jailbreak was reported by Geohot in a tweet only a couple of hours ago. Luckily for us; the iPhone Dev Team is on it too.
As of now; he doesn't provide any video demonstration rather only a photo which shows an iPhone with Blackra1n and Cydia installed on it which clearly indicates that the iPhone 4.0 Firmware has been Jailbroken.
Apart from this; Geohot also promises an Unthered Jailbreak for iPhone 3GS and iPod Touch 3G in the near future, along with a Jailbreak for iPad on Firmware 3.2. No ETA provided.