Originally Posted by halfspeed
There are mountain bikes and there are "mountain" bikes. The latter are sold at Wal-Mart and have stickers that say "not to be used off-road" on the seat tube. If you count the latter along with real mountain bikes, then MTBs dominate the market. Otherwise, my LBS says they've noticed a strong trend away from MTBs and towards road bikes.
I'm 100% certain that if one were to not count the sales of the big box stores, road bikes would currently dominate and probably by a good margin. They've made a big comeback in the past few years. Real MTB's now offer a good bang for the buck value though thanks to the reversing trends in sales.