The newly released iOS 4.1 Beta 2 is jailbreakable using the existing version 0.9.5b5 of Redsn0w. However this Redsn0w jailbreak is supported on iPhone 3G and iPod touch 2G only. Here is the update on this from the iPhone Dev-Team blog:
If you’ve updated to 4.1beta2 on iPhone3G or iPod2G(non-MC), you can use this redsn0w from June to jailbreak. Just point it at the 4.0 IPSW (the public one, not the developer GM one).
(Yes – point it at the 4.0 IPSW even though you’re at 4.1beta2).
Don’t do this if you need the ultrasn0w carrier unlock!!
Note 1: This won’t hacktivate your iPhone3G – but for those with legit access to the 4.1beta2 this shouldn’t matter. Please don’t pirate Apple software
Note 2: If you’re at 4.1beta2 you’ll probably experience network problems after jailbreaking. Until a new version of redsn0w is released, you may need to rely on afc2 access (such as FunBox-like file browser programs) to manipulate your filesystem.
Note 3: This is recommended only for power users who can deal with the problems that will pop up as the JB apps fall further behind the beta firmwares.
So all you need is the latest iOS 4.1 Beta 2 firmware for iPhone 3G or iPod touch 2G (non-MC Models) and you can jailbreak it using Redsn0w on Windows and Mac. Again, this is strictly for iPhone 3G and iPod touch 2G (non-MC models) only. If your iPhone 3G relies on an unlock, you MUST STAY AWAY from iOS 4.1 for now.