MTB folks are American good 'ol boys and think in inches. We have George Bush and 26 inch wheels. Now, we have 29" wheels as well. Roadies are Euro-loving, tour Day France watching fools who have John Kerry and 700c wheels. Those, are really 622 (not 700). Plus, the c implies cm, but that would be huge. It should be m for mm.
Of course, GW is a liar too. His wheels are not really 26" and 29", they just grow to about that size with the appropriate size tire. GW is currently trying to get pi adjusted so that his cyclocross wheels can be 29er as well.
29er wheelsets and 700c wheelsets are the same diameter and have some overlap, but they are generally designed differently. 29ers will often be disc friendly or disc specific, they will generally be on mtb hubs, 135 spacing, beefier components, and so on.
Al Gore invented 32" wheelsets. They will be out in 50 years, if the earth is still cool enough.
Half of what I just typed is meaningless drivel. The other half may be, too.