Here are links to two of Trek's lines. The different prices reflect better components, but for commuting I think even the lowest-priced one would serve you very well. As you go up in price the bikes get lighter (not very important) and the components and wheels get a little more durable (even the low priced ones are pretty durable though).
The first one is (I think I heard) their most popular line. It has a little sportier geometry, where the rider is leaned over a little more for speed. My wife has the 7.3 fx, and she's been very happy.
http://www.trekbikes.com/us/en/bikes...ness/fx/models
The second one is similar, but the rider isn't leaned over as much. At commuting-type speeds that doesn't usually make any difference. These already have fenders, but installing fenders is cheap anyways.
http://www.trekbikes.com/us/en/bikes.../allant/models
All the big bike companies have similar lines, and there's little to no difference in value. I just linked to the Trek page because I happen to know the web address. But you could look at Specialized and Giant too-- they'd have similar bikes with minor differences. I'd say these kinds of bikes are as good a place to start as any!