HI,
I have an @mac.com name I use in iChat and Messages as an AIM Screen Name.
IF I remove my current ID (Use the Account Details button and then Sign Out Button and enter new Into) I then get the message that there was an error and it "failed the verify"
If I then use the "Remove Email" option button it goes back to the "account" screen with the spinning cogwheel still going.
The only way to stop that is to use the Account Details button and Sign Out button inside.
I have done s brief search for mac.com and iMessage and I am not seeing any Apple Documents show up (not in Google searches or a search on Apple)
I tried with a second @mac.com name that I had just checked the password on the Apple ID site. (in fact I changed it)
When I get these failed accounts It reads that I am in the United States and trying to set this to the United Kingdom does not seem to work.
Adding an Address to the Info on the My Info Apple ID site did not seem to help either (Although it does register the fact the account is a UK one).
I am now at a loss as to what to do next.
This Messages seems like it would be helpful in that it appears to offer a Link
However there seems no other option about how to contact Support.
Little Snitch tells me that a "normal" login contacts three servers at the iMesssage end and the failed ones only make it though the first.
This means I can recreate this but also find the "Standard" Fixes do not seem to work.
EDIT
However I would also look at this thread as the person who go the points had posted they contacted Apple Support directly and got it booted up the chain to a Senior rep.

8:38 PM Monday; July 30, 2012
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously
Message was edited by: Ralph Johns (UK)