Anyway, I've been biking in Philly a couple of years now and am wondering if those of rest of you who live or have lived here find cycling here to be unusually treacherous - esp. on the streets of Center City. Don't have much prior experience with riding in a major metropolis.
I ride in center city Philadelphia pretty often. The streets aren't very wide and the sidewalks are crowded, so in my opinion the best option by far is to pick a lane and ride right in the middle of it. Drivers may get mad, but with all the stop signs, traffic lights, and stopped vehicles you won't be slowing them down much. There is no room at all on those streets to ride at the edge of a lane in order to let people pass you in your lane.
I don't know how much you've heard about vehicular cycling, but I don't think any of people's reservations about riding-like-you're-a-motor-vehicle apply in a place like center city. Because traffic is relatively slow, cars and motorcycles and bicycles all have to behave in nearly the same ways on the roads. So if you want to make a left turn, you should do it from the left lane. If you want to go straight or right, ride in the middle of the right lane. If you want to change lanes or turn, i suggest sticking your left arm straight out to signal left and right arm straight out to signal right at some point before you do it (and before you need to use your brakes), but of course look to see that it's clear as well.
Most of center city's streets have an actual traffic speed of about 30 or 35 mph max, which should feel pretty safe if you're going 15mph yourself. J.F.Kennedy Blvd. and North Broad St. often have people going really fast, and if you don't like that you might want to avoid them. Vine St. too, i suppose.
also, don't jump forward on a new green unless you can see for sure that nobody's running a red light... but you probably knew that already.
I feel much safer in the dense, slow traffic of center city than in the fast-moving traffic you find 15 miles away. It takes some practice to feel comfortable, but just a confident attitude and basic road-user's common sense are pretty much all you need. Don't let yourself worry about somebody hitting you from behind while you're riding as assertively as one ought to drive a car, because that type of accident is very rare. It's much more common for people to get hit doing stupid things at intersections such as behaving unpredictably (which can happen when you act timid or scared).