[z2amiller]

Good advice all around. If you wear a backpack or messenger bag, you'll end up with an awfully sweaty back. Getting rear panniers will help a lot, especially since you'll probably want to carry a change of close with a 15 mile commute.

Bigger tires are better - I ride on Rivendell Jack Brown tires which are ~33c. The larger tires are a smoother ride and can handle debris better. Also make sure your tires are kevlar belted. City streets, even in bike friendly cities, collect a lot of broken glass and other sharp debris along the bike lanes.

Make sure you have a good lighting system, come wintertime it will likely be pitch dark on at least one of the legs of your commute. I just got a NiteRider MiNewt 250 which is nice because it is a self contained unit and has mini-USB as its charging input (So no need to carry a specialized charger, can easily charge while at work or even in a cafe). A friend recommended the Novae Cyclone 900, less convenient but brighter than the MiNewt and cheaper. http://light-reviews.com/forum/viewtopic.php?f=29&t=897