Just do the 60 day .Mac trial now and it will go to MobileMe, then you'll be able to keep your .Mac email address after the transition.
I think that they need to release Mobile Me and software 2.0 a few days before July 11. On July 11, all of the servers will be so jamed that between activation of your new iPhone and getting setup on Mobile Me it may be tough getting on. If it were launched say 3 days before and a early release of 2.0, people could get everything set up and ready for iPhone 3G.