Short version:

You can re-use redsn0w v0.8 we released a few weeks ago to jailbreak today’s 3.0.1 update.  Just let iTunes update or restore you to official 3.0.1 then run redsn0w.  The only “trick” is that when redsn0w asks you to identify the IPSW used, point it at the 3.0 IPSW instead of the 3.0.1 one.   After the jailbreak, reinstall ultrasn0w 0.9 if you need the unlock.

More details:

The 3.0.1 release is a “branch” from 3.0 that occurs (code-wise) before all the 3.1 betas.  The programs redsn0w needs to change for the jailbreak are identical when you compare the 3.0 and 3.0.1 versions.  It seems pretty much the only changes Apple made were for the SMS bug, which affects programs that redsn0w doesn’t touch.  That’s why you can re-use redsn0w 0.8 on 3.0.1 even though it was written for 3.0. 

And since 3.0.1 doesn’t touch the baseband either, ultrasn0w 0.9 works for those needing the soft unlock.  Just install it from the repo666.ultrasn0w.com repository using Cydia as usual.

We’ll at some point fix redsn0w to recognize both 3.0 and 3.0.1 IPSW’s, but really that’s the only change that would be made to it.  Everything else would be identical, so there’s no need to wait for the “proper” version that recognizes the 3.0.1 IPSW as valid.