According to the iPhone Dev Team developers, after public release the iOS 5.1 - you can't downgrade to iOS 5.0.1. If you are waiting for 04.11.08 Unlock - Don't Update To iOS 5.1.
Dev Team Does not Recommend to Upgrade to iOS 5.1:
Stay turned to UnlockBoot and wait for jailbreak iOS 5.1.
- Soon after iOS 5.1 appears on Apple's public servers (when iTunes starts to offer it), Apple will stop signing 5.0.1 SHSH blobs.
- If you have an iPhone 4S, 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.