First, determine your budget. Next, decide upon the type of terrain you'll be cycling on and the distance you'll be traveling on it.
Bad neighborhoods and college campuses are black holes for decent bikes. Chances are, if you leave the bike unattended for just a few minutes, you'll never see it again. Therefore, I would either shop online or used. The problem with buying used, is the fact that you have to know something about bikes, or else you could get taken. Buying online can be great, provided that you order the right sized bike.
Another thing to consider would be the flatness of your terrain. If your commute is mostly flat, with no really challenging hills, then a single speed can be less expensive and require less maintenance.
The Nashbar Bull single speed seems like a really good deal!