What kind of terrain is your commute?
You can carry a backpack for a 2.5-mile ride, but recalling my student days, I would not want to ride with 30-lbs of books on my back. I use a rack-mounted rear basket, and set my backpack in that. Other people use panniers. It's a matter of preference, but probably you want some kind of load-carrying capacity.
If you will ride all year you probably will want fenders too.
So, I would look for something that can take a rack and fenders.
It sounds like a mountain bike with 26" wheels would be good. I would look for one without suspension, as typical fat mountain bike tires will give a smooth ride all by themselves. [edit: Slick tires will go the fastest (you'd have to get them separately), but knobbies at max pressure aren't actually that bad.]
It is also good to have a bike that is more . . . theft resistant in the aesthetics department.
I'm thinking go to a decent bike shop staffed with knowledgeable people and get a used bike. You'll get a good, serviceable bike for cheap that way and can also get good advice.
Last edited by beezaur; 08-22-11 at 12:43 PM.