Bigsung, Check out this thread, lots of other people have turned their old mountainbikes into commuters:
http://www.bikeforums.net/showthread...light=hardrock
Yeah, the threads over 3 years old, but it shows you what you can do.
Consider adding lights, it will be getting dark early sooner than anyone would like, fenders are great for helping keep the nasty road grime, oil, brake dust off you and your bike when it rains.
I've been commuting on mine for 5 years now.