Having one of those moments where something that I could tell someone to do, I can't actually remember for myself. As far as I was aware, in Mail, the order of the accounts specified which was the default one (i.e. the one at the top of the list). I have 3 email accounts. For some reason, my Gmail account is always the default one. I want my MobileMe account to be the default. I know I'm going to get a face palm response with the most obvious solution, but I just can't think right today for some reason.