If you’ve upgraded to iOS 5.1 and were hoping to downgrade to 5.0.1 for whatever reason (such as to have access to an untethered jailbreak), you’re now officially out of luck as Apple has now stopped signing iOS 5.0.1. The only possible way you can downgrade now is if you have an A4 iDevice with a custom 5.0.1 IPSW created using iFaith or redsn0w. A5 iDevices are incapable of downgrading altogether as they aren’t compatible with the limera1n bootrom exploit that is present in A4 iDevices.
If you haven’t upgraded to iOS 5.1 yet, and an untethered jailbreak holds any value to you, it is highly recommended you avoid upgrading until an untethered jailbreak is confirmed for the new iOS version. A5 iDevices are even more at a loss when updated to iOS 5.1 as they aren’t even capable of the same tethered jailbreak that A4 iDevices are due to the lack of a bootrom exploit at this time. Many Cydia packages are not even compatible with iOS 5.1 right now, as their developers will need to update them and may choose not to do so until an untethered jailbreak comes around for iOS 5.1 (if it even does).
If you have updated to iOS 5.1 on an A4 iDevice and don’t have a custom iOS 5.0.1 IPSW to downgrade to, you can perform a tethered jailbreak using redsn0w 0.9.10b6. Everyone else will have to wait for now until more progress is made on a bootrom exploit for A5 iDevices and an untethered jailbreak for all iDevices.