Ok so some people have reported of youtube not working after they jailbroken their devices. There is a very simple fix.

This is usually a case from using blacksn0w to unlock your phone.

1. Launch Cydia -> Click Manage -> Click Sources -> Click Edit - Click Add and add url "http://cydia.iphoneil.net" without quotes
2. Let cydia refresh and launch it back up
3. search for "Push Fix & Youtube" and install it, that's it.