I started out 3 years ago at 275 and rode a mtb bike for one year. I found that riding more than 30 miles a day was hard on my lower back but I got down to 255. The next year I went to a hybrid (Gary Fisher, Nirvana) and put in around 40 to 50 miles a day (this bike weighs 44 lbs with 700X37 tires on it). Long rides gave me sore shoulders but I got down to 240.
The next year I bought a Trek 520 and rode it unloaded with 700X28 tires on it. Great ride for a heavy rider. I finished in November at 225 lbs. and was really in shape cuz the drop bars really worked my abs and biceps. The relaxed ride was much easier on my knees and back than either the mtb. bike or the flat handlebars. Also, a road bike is much more efficient. I've been doing 50 to 60 miles a day (wearther permitting) and it does not make me sore at all.
I've just ordered a custom fast touring frame and that should keep me happy for a while.