After a few very dramatic days in the jailbreak community, geohot has come out of nowhere to release limera1n. It’s a bootrom-level jailbreak that works on the iPhone3GS, iPhone4, iPod touch 3G, iPod touch 4G, the iPad, and (technically) the AppleTV 2G.
DO NOT USE LIMERA1N IF YOU USE THE ULTRASN0W CARRIER UNLOCK — wait for PwnageTool to incorporate the limera1n exploit. This is so that you can avoid updating your baseband and losing the unlock (possibly forever).
Limera1n uses a different exploit than SHAtter, and in fact covers more devices. Although some may question geohot’s dramatic and competitive style, he obviously does have considerable skill pulling this together in just over a day (although he’s had the underlying exploit for months). Credit also goes to @comex, who provides the untethered aspect of limera1n via another one of his growing list of kernel hacks.
The release of limera1n has (hopefully!) averted the burning of 2 bootrom holes at once (both his and SHAtter). Releasing SHAtter now would be a complete waste of a perfectly good bootrom hole in light of limera1n, and so it can be held until Apple closes limera1n’s hole. While there’s no guarantee that Apple won’t also close SHAtter by then, it provides a ray of hope for devices after Apple’s bootrom respin. Meanwhile, look for alternate implementations of the limera1n exploit in greenpois0n and possibly other tools too.
Limera1n does have some issues, and geohot typically works these out in subsequent (and rapid) releases.