I had the pleasure of buying a new bike this weekend (my last one was stolen in June). Rather than replacing it with a mountain bike, I got a commuter. I've heard they're great for riding around town and can make the ride-to-work thing a lot faster. I'll probably buy a mountain bike too, but I'm going to hold off until next year for that - the Bus will be taking over as my top time-consuming hobby in the near future.
Update: More about the Giant FCR3.
Update 2: I set a new record on the ride to work this morning: 23 minutes! I took the long way to get home and it took 45 minutes. My goal is to reduce both times by 25% by the end of summer.