Hewitt beating Federer is a big surprise indeed!
I think the biggest adjustment for Federer over the next few years is to be slipping down the rankings and not winning EVERYTHING....
I think he could still play until his mid 30's - Agassi won his last Grand Slam at 35 - but whilst I think Federer will win at least a couple more Grand Slams I don't think he'll ever beat Nadal in a Grand Slam final again - injury to Nadal aside of course.