Today the solution was finally launched untethered jailbreak for iOS 5.1.1 called Absinthe 2.0 and hundreds of thousands/millions of iDevice owners are now rushing to jailbreak their own devices. This influx of users who have jailbroken and access Cydia has generated functionality problems of some repos, so when opening Cydia there are very high chances of receiving HTTP 1.1/500 errors or package download errors, but don't think that there is a problem with you.
Unfortunately, the servers that hold the Cydia repos can't handle the traffic and you have to wait a few days until things calm down and Cydia will work the same as it did until today.
