Another compounding issue is that for years you could use xxxx@gmail.com as a valid apple ID but with the release of iCloud they now force you to create a xxxxx@me.com address for some of the features (although I can't remember which one). This means that a non-apple email address on your apple id now means you're a second-class citizen.

To make it extra confusing, apple uses your email address as the way to identify the apple id but then also give you a way to change the email address, but only if its not from a me.com address:

http://support.apple.com/kb/HE40