iOS 5.0/5.0.1 could be extremely easily jailbroken using the redsn0w developed by the Dev Team but with iOS 5.1 the situation is different because the latest version of the team program does not work. I tried to jailbreak an iPhone 3GS using redsn0w 0.9.9 beta 8 and an iOS 5 ipsw and the process got stuck on the terminal with a not very nice error. Unfortunately, it seems that Apple has blocked the redsn0w functionality again in iOS 5, but it was expected that this would happen considering that Apple usually takes such measures with important versions of iOS.
The good part is that redsn0w will be made compatible with iOS 5.1 again because the bootrom exploit in limera1n allows it to jailbreak old iDevices regardless of the iOS version installed in them. When a new version of redsn0w appears, I will let you know, but anyway iOS 5.1 should not be a "target" for you.
