Unable to verify “@mac.com” because it is already in use by another Apple ID.

I'm having the same problem. I've verified my Apple ID, and it is connected to my @mac/@me email account, but after I log in to facetime using that Apple ID, when I input that email account on the next setup screen, it gives me the error: "Unable to verify “username@mac.com” because it is already in use by another Apple ID." Interestingly, when I use the @me.com version of the email address, it returns the error: "An error occurred while verifying “username@me.com”.

I tried to use my gmail email address, and that worked (after I verified it and associated it with my apple ID. But why the heck can't I use my @me.com address???