I used a previously registered and then dropped MobileMe address as my iTunes ID for a long time. You just don't have access to that email anymore, so definitely do as alust2013 suggested and change the account email address, but your login and password will continue to work long after you've dropped your MobileMe account. Better yet, if you ever want to get back that address, you can renew it on MobileMe in the future, though of course it won't have your emails still stored. The address is permanently yours, though, even if you drop the service.
jW