It seems Microsoft have taken new steps to stop people sharing their game saves and are now including a revocation list containing ID’s that will no longer be accepted on different Xbox 360 consoles than the save was created on.
I’ll be doing a little bit more research on this and posting my thoughts over at gothi.co.uk in the next day or so but in the mean time I’ve knocked up Revocation ID Checker to quickly check if your saves contain a blacklisted console ID.