Jailbreaking your iDevice does void your warranty. If at any time you need service performed on your iDevice by Apple... you can do a restore via iTunes to wipe all traces of the Jailbreak. As far as updating after a Jailbreak... yes, you can update to new iOS versions as they are released. However, when you do this you will loose the Jailbreak that was good for iOS 5.1.1. Until a new JB is released for the newer iOS version.