Please see the forum Compendium. It will have some good advice.
It is SO a matter of taste. I dont agree with the sentiment “any piece of junk will do.” If you are going to put in the miles, you are going to want a vehicle that can perform. I own a DREK and a Cannondale. The DREK is terrible. It rides like I am dragging a dead zombie. The Cannondale is fabulous and seems to simply fit me.
My Cannondale is an urban bike: A Cannondale Bad Boy. sort of like a hybrid, it is more urban in design. Stronger than a road bike, it can take a beating and support racks. But it still has somewhat of an aggressive design, allowing for good speed – I am not sitting upright like a giant parachute. It’s an absolutely wonderful bike.
And for the same reasons, I agree with the cyclecross recommendations. These bikes can take a beating, fit you well, and give you speed. The Specialize Cyclecross from this last year looked fantastic. While it has drop bars, I love the additional brake levers up in the flats of the handlebar, for upright urban cycling when you want to be more able to see the traffic in front of you.
Someone else on the forum just bought a space horse from All City. A steel road bike that looks like a dream. Steel is going to give you a lot smoother, more comfortable ride. For this reason, many commuters seem to love the Surly’s. We test road the Long-Haul Trucker. Magnificent bike. Probably a more comfortable ride – essentially a touring bike design – with the benefits of a road bike position. But my kid elected for an aluminum road bike because, well, he’s a kid and he wanted a fast machine.
Are you going to have secure parking? I have a secure indoor garage. This means the cyclists at my work frequently ride *nice* bikes.
Check out Phoenix Bikes. They are a youth organization that sells bikes in Arlington. They are going to have their big sale at Big Bear Cafe…. This saturday. Great group.
Check out Ye Old Bike shop in Arlington. They also show up at the farmers market at Courthouse on Saturdays.
Watch Craiglist. Good deals there.