Ever since, Apple released iOS 5.1, there were reports about Apple stop signing in iOS 5.0.1.
The reports alarmed those users of iPhone and iPad who after upgrading to iOS 5.1, wished downgrade, back to iOS 5.0.1 due to one reason or the other.
People with jailbroken iPhone and iPad were more concerned. It is impossible to downgrade now.
Now we can confirm that these reports are indeed true.
If your iOS device (or devices) is jailbroken, you must not update it to iOS 5.1. As expected, the latest software update for Apple’s mobile OS breaks both the Corona untether and the Absinthe exploit.
Even though Apple released iOS 5.1, last week, alongside the announcement of its new iPad, it continued to sign 5.0.1 firmware over next two days hours or so. Now it has stopped signing iOS 5.0.1 and the reason for that is obvious. Apple wants you to upgrade.
However, those who have an A4 device that updated to iOS 5.1 can still downgrade back to 5.0.1, provided they have done the necessary prep-work. However, those having an iPad 2 or iPhone 4S, have no way of doing this.
Currently, there is currently no jailbreak available for iOS 5.1. Nobody knows when it will be available. Users with older (A4) devices can try to jailbreak via RedSn0w, but it is tethered.
Source: iDownload Blog