This morning (16 December 2011) a user came and complained that he wasn't able to login into his iTunes account nor on this iPhone nor on the PC. He was worried account has been hacked or otherwise compromised. I quickly checked my own account and wasn’t be able to login either.
Error in an iPhone was "The Apple ID you entered couldn’t be found or your password was incorrect. Please try again.
Trying to login into Apple account on a PC the error was: "Fatal internal error occurred.
This seems to be quite widespread issue at the moment, with user all over the word not able to login into their iTunes and other Apple accounts with Apple IDs.
Apple support acknowledged the problem, but there are no workarounds or ETA. That's the "beauty" of Cloud services – if something goes wrong all you can do is sit and wait…
Problem was resolved later the same day. Seems that the glitch was caused by new Apple's music service iTunes Match
Apple experienced similar problems with new service launches before, so I will leave this post online - it's not the last time Apple launching a new service...