The above picture shared by Geohot on twitter, shows blackra1n running on iPhone OS 4. There is no definite ETA (Estimated Time of Arrival) for blackra1n jailbreaking tool for iPhone 3.2/4, but it must be ready by end of April for sure.
Geohot had managed to jailbreak iPad 3.2 a couple of days back. he had posted a tweet sharing the screenshot of iPad running on the verbose mode. To sum it up, we can expect the blackra1n to jailbreak iPhone 3.1.3/3.2 later this month, once Apple releases iPad 3G. Expect the cat and mouse to become much more interesting in the coming days.
Update: Guide to Jailbreak iOS 4.0 using PwnageTool is here. BlackRa1n tool to jailbreak iOS 4.0 is yet to be released.
Update 2: Guide to jailbreak iOS 4.0/4.0.1 running on Iphone 3G/3GS using jailbreakMe 2.0 Star is posted here.