I agree with drunkle, ignore the steel vs. aluminum nonsense, and don’t bother screwing around with electric motors and the like. I also recommend against getting a mountain bike unless you’re actually going to do some trail riding.
I went through this about 2 years ago, and I decided on a Giant FCR-3. This is a pretty decent commuter/hybrid bike. It has a fairly upright posture so it’s comfortable to ride, it’s fairly light, and it’s fairly fast. I actually did the Tour de Palm Springs (100 miles) on mine in 2008. It’s a very decent bike for the money.
Add a rack and a bag in the back, and you’re set to go. I paid a little over $300 for mine at Bicycle Warehouse. I did look at the Treks, but bang-for-your-buck, it’s pretty hard to beat Giant. I do recommend that you get some bar ends, as the straight bar gives you pretty limited hand positions which gets tiring on long rides. Also, be sure to invest in a good helmet, lights, bell, etc.
I’ve since purchased a Specialized carbon-frame road bike that I use for longer rides, but I still ride my Giant FCR-3 all the tine. It’s great for anything up to about 40 miles.
