Originally Posted by pinkrobe

If you run STI shifters, you can use the Avid [BB7?] Road discs. They modified the amount of cable required to pull the brake, so it works with standard road brakes. If you stay with Shimano shifters and derailleurs, you should be able to swap road and mountain back and forth no problem. Ultegra shifters work fine with XT ders, so you can use damn near anything.

The stem could be a pain, but you should be able to find some cheap hi-rise stem at your LBS for $20. It might be tougher if you stick with the 31.8 OS bars. The older 1" stuff shows up more often in the weird sizes.