I have my own domain hosted elsewhere and use domain mapping here at wp.com. I got a notice in my dashboard a few days ago about it being time to renew my domain mapping, which I promptly did. My upgrades now show that my domain mapping expires 1/5/09. Tonight, I happened to notice that my url in the address bar was no longer my mapped domain but the old wp.com url. Checking in my dashboard, I saw that it had listed that my domain expired 1/6/08. Now remember - my domain is hosted elsewhere. As the result, even though I have paid for domain mapping, wp thinks my domain is expired and reverted back to the old url.
I don't have a clue as to why this happened because wp.com shouldn't even be trying to track the domain itself when it is not hosted here. I have sent an email to support asking for a correction but I wanted to post this here just in case somebody else runs into the same problem.