Touring bikes (520 stell, beefier wheels, etc) are generally accepted as the best bet for us uberclydes. I've definitely had issues with my mountain bikes, but have yet to break a frame. Mostly I just destroy bottom brackets. I'm riding a Randonee now, and it feels great. Doesn't accelerate as quickly as a stiff aluminum frame, but feels more solid - if that makes any sense.
BTW, plenty of 350+ pounders are getting lots of miles out of many different bikes out there. It can be done!