I commute on a road bike.
I had an aluminum road bike, but eventually bought a carbon fiber road bike. Once I started riding the carbon fiber bike on the weekends, I decided the aluminum bike would become my "commuter". But, eventually, the cf bike became so much more enjoyable to ride, and I got rid of my aluminum bike. Now, I commute on my Giant TCR C3 and I love it! Of course, I don't leave it locked up outside because I'm afraid someone will steal it. Either way, though, I think commuting on a road bike just makes sense for me since I also ride on the weekends and don't need to own two bikes.
I say, buy the bike that makes you happiest because you're more likely to get on and ride it.
I'm Another Cross Enthusiast
I recently switched to a mountain bike with slicks on it to a cyclocross bike, (28 mm slicks) and I've never looked back. A cyclocross is versatile, tough but comfortable, and lets you go fast on occasion as well. I've received many admiring comments on my cross bike, so I too recommend considering a cross. Just avoid disc brakes if you do choose one -- they're more hassle than they're worth!