I've been using legitimate iTunes gift cards bought from actual Apple Stores in the US and was absolutely gobsmacked when I found the same message: "Your iTunes account has been disabled for security reasons"

A quick check on the Apple support website for iTunes suggested that you should reset your password. Before trying that, I tested my MobileMe account and found the same error message as well so they're both linked. Reset my password and I'm happily downloading again

BTW: the "security reason" is apparently because the wrong password had been entered multiple times. Since I have my password saved in my Mac keychain, I KNOW I did not type it in wrongly so I have no idea how this could've happened. Perhaps someone/ppl trying to access my account? Anyway, to those affected, try resetting your password.