Why cant I use my @me email as primary email address on my apple ID?

My current Apple ID is using my yahoo email address, but I want to delete my yahoo account and only use my @me address. When I try to change it, it says "You cannot create an Apple ID using a MobileMe account. If this is your email address, it is also your Apple ID." I tried deleting and creating a new account but it didnt work. please help!

Thanks!