used to do the spare wheel thing when I commuted on my MTB. One set with slicks for the weekdays, one set with knobbies for the weekends. Same rims, same hubs, same cassete. Never had a problem with the rear derailuer. My biggest problem was that I had discs. It seemed that I could never really get the rotors to sit clean in the pads. The tollerances were too close. PITA. Finally solved the problem by getting a road bike.

I will make you a good deal on some Sun rims on deore hubs though!