The new iOS 5.1 build does add a couple features and probably bug fixes, but the bigger importance is what it means for the jailbreaking community. The iPhone Dev-Team is warning jailbreakers to avoid iOS 5.1 which will likely be released shortly.
"In all likelihood we'll see the GM "gold master" version of 5.1 this week. DO NOT UPDATE TO 5.1, because you may lose your jailbreak! The rest of this post details the subtleties with this rule, but if there's only one message to take home, it's the overall "do not update" message! "
Now for the nitty gritty exceptions:
● 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.
So to summarize once again, DON’T UPDATE! You have been warned.