Matt,
Listen to Pete, he knows what he talks about. Fortunately here in Iowa, I do not have to worry about bad neighborhoods or even long commutes (I could bike the circumference of Iowa City in less than 20 miles). However, I have lived in larger cities where I would be apprehensive about biking in certain areas. From what JonR says, he could probably give you some good advice on big city riding. My biggest worry here is watching out for the roving bands of factory farm escaping hogs.
I do commend you for considering commuting to work. I would consider a 15-20 mile commute a long ways, but if that is what I had in front of me, I would do it with a smile.
