Rode a 94 Fuji Discovery mtb for the better part of 5 years 10 miles/urban each way. Rigid frame and fork. Took the knobby tires off in exchange for street tires. Rear racks w/a waterproof set of panniers and a trunk bag should give you plenty of room for anything you would want to carry. Depending on your job you may want to get a Commuter Garment Bag from www.nashbar.com. It's not waterproof, though. Plastic bags would solve that problem.

You'll need fenders, lights, frame pump, tools, etc. Best to go to your lbs and have them help you set up your bike for commuting. Just making the decision to go car-light is a big step. The key to riding in traffic is to not panic, be assertive and point clearly where you're going. As you get to know your commute things will become more automatic. Anyway, congrats! Post back on how you're progressing.