I think you'd be better off buying an MTB designed around 24" wheels, if you have that option. Putting those on a bike designed for 26" wheels will result in a much lower bottom bracket, which will increase the likelihood of your cranks striking the ground during turns- especially sharp ones at low speed, which people do a lot of in mountain biking. All your gears will end up being a lot lower too; you'll be able to pedal up anything, but you'll spin out at a much lower speed.
One thing I'm not familiar with is the axle width on 24s- anyone know if a 24" wheel will fit in dropouts designed for 26"?
Last edited by rnorris; 07-30-09 at 11:39 AM.