29" wheels - what advantage do they provide over smaller wheels?
Regarding road bikes, the 29" rim size is actually the same as standard 700c wheels used on road bikes -- only the tire is different (larger).

One difference that I don't see mentioned often is that 29" vs 26" wheels change your effective gearing.  If you have two bikes with the same gearing but one with 29" wheels and one with 26", the larger wheel will have a higher gearing overall because of the larger wheel diameter (one turn of the pedals move you farther down the trail).  This means that the 29er can help you reach a higher top speed, but the 26er will have a lower climbing gear.

Also see the comparison on Wikipedia.