The Chronic Dev team issued a simple warning to both veteran and amateur jailbreakers during the Apple iPad event yesterday. Along with the new iPad announcement Apple released iOS 5.1 for the iPhone 3GS, 4 and 4S, the 3rd generation iPod touch, iPad and iPad 2. iPhone 4S and iPad 2 users should not update to iOS 5.1 if they want to keep their jailbreak. Read the Dev Team's complete warning below.
Soon after 5.1 appears on Apple’s public servers (i.e. iTunes starts to offer it), Apple will stop signing 5.0.1 SHSH blobs.
If you have an iPhone4S, the basic rule above is really the only rule: you cannot restore back to 5.0.1 once the 5.0.1 signing window is closed, no matter what (even if you saved your SHSH blobs).
If you have an iPad2 with saved 4.x hashes, you can in fact downgrade to that 4.x but you won’t be able to get to 5.0.1 once the 5.0.1 signing window is closed (even if you saved your 5.0.1 SHSH blobs).
If you have a device earlier than the iPad2, you can downgrade to whatever version you want, as long as you have saved SHSH blobs for that version. You’ll need the assistance of geohot’s limera1n exploit with tools like redsn0w to get into “pwned DFU mode” and bypass the downgrade restriction.
The Dev Team also mentioned that non-A5 users "can test the 5.1 jailbreak waters if they really want to." You can download redsn0w 0.9.10b6 for OS X here or Windows (be sure to run in Administrator mode) here. However, this is a tethered, not untethered, 5.1 jailbreak for non-A5 devices. Here's the Dev Team's warning for this version:
If you use ultrasn0w for your carrier unlock, be sure to use a custom IPSW to get to 5.1 first! Don’t ever restore to a stock Apple IPSW! Use redsn0w’s “Custom IPSW” button to create a NO_BB_* version of the 5.1 IPSW and restore to that instead of the stock one. (That option is available only to 3GS and iPhone4-GSM owners.) ultrasn0w itself will be updated for 5.1 in the next few days (same baseband support, not 5.1’s baseband).
If you’re lucky enough to have an old-bootrom 3GS, this jailbreak is actually untethered (redsn0w will figure that part out automatically).
While we were at it, we added @pod2g’s steaks4uce exploit to support MC models of the iPod touch 2G (whose last firmware was 4.2.1). So now redsn0w will auto-detect and jailbreak both MB and MC versions of that older device.
iBooks won’t work until a future update of redsn0w