Hi Joey,
I have confirmed with iCloud technical support that there are DIFFERENT kinds of Apple IDs. In my case, my old Apple ID is an incompatible Apple ID for iCloud, but it works fine with iTunes, iOS Dev, Apple Sales Web, iTunes Match and the App Store. It just doesn't work for iCloud, or as I will now refer to them as iClowns.
Originally iCloud support said that my Apple ID was "locked" by iTunes. But later they said "So sorry, that Apple ID is incompatible with iCloud. Here are three free iTunes songs for being bounced around tech support circles." because iTunes support could not help me and bounced me BACk to iCloud support.
Here's my suspicion... I noticed that when I finally gave up and migrated my old Mobile Me (old mac.com account initially) account to iCloud that iCloud ACCEPTS my user@me.com log in and password, but when you log into appleid.apple.com to edit your account my Apple ID actually shows up as USER@MAC.COM. So what I THINK is happening here is that Apple is actually storing your REAL Apple ID as whatever it was when you set it up and all of us who get the unsupported message have an alias of an email laddress like gmail or me.com, but our actual Apple IDs in the system are the older non-email based Apple ID.
How. In. The. ****. Could. They. Let. This. Happen.
It tells you a little about how Apple is organized internally. There are a lot of silos at Apple. One group has ownership of something and they will not give it up. They do not neccesarily talk to each other and they don't test across departments for compatibility.
Silly people.
The only bright news in this all, the iCloud support people really looked hard for a solution but in the end had to send me away with nothing.
YMMV, good luck - hope everything worked out for you in the end.
K