I was in LBS the other day looking at bikes and discussing what I should buy and he said a mtb with 29 inch tire.
See I told him I wanted to ride road,but as former Bmxer and mt biker I may want to ride trails also.He said for me to get the 29 inch tire mountain bike and put road tires on it,to be able to put miles on road,also a pair of dirt tires and switch off and on.
I am not worried about going fast on the road,I really as of now, want to use the bike to get in shape.
My real problem is If I really get into road biking,I'll buy a real road bike,
But if I also enjoy riding trails will I be able to enjoy them on this type bike.
I do not want to have to go out and buy two new bikes,in a year or two.
That's probably the most active 29-inch community you'll find online, and the interest in that particular forum has just been exploding as of late.
I've owned a 29" hardtail for a few years now, and I just love the damn thing. I'm confident I'll own a 29" full suspension in the coming years, and there are quite a few now to choose from.
However, specifically to address your question, **many** 26" mountain bike frames and forks will fit a 700c road wheel and tire. With a disc brake setup (and some appropriate shimming of the rotors), this would make wheel swaps a snap.
On the other hand, there are quite a few 26" slick tires that will fit the bill as well.
So really, you have many choices, but especially if you're a bigger guy, look into the 29" bikes to satisfy your curiousity.
You definately need a mtb but I'm not sure what you'll end up doing with it. Riding the road and trails, go with a xc bike with light components. However, if you ever want to come back to the world of mtb'ing trails and bmx style dirtjumps then you better just get a trail bike with 26 or even 24" wheels.