Yes, keep your wallet in your FRONT pocket. There are pickpocket on the subway and in crowded places popular with tourists like Times Square. Same with cell phones, iPods and other valuables.
As for backpacks, two reasons not to wear them especially on your back are those same pickpockets will slit them open to try and find something of value, and two, if you aren't used to wearing them on a crowded subway, you are bound to hit someone in the head with one.
Another tip: when going down an escalator with luggage trailing behind you, keep an eye on your bag. I've seen pickpockets zip things open looking for stuff while the owner was facing front. At that point I usually yell and smack them with my umbrella and they scatter like the cockroaches they are.
Don't let this alarm you though. New York is still one of the safest cities in the country now. We all walk the the streets and ride the subway carrying laptops, iPods and shopping bags just like everywhere else. Even backpacks. The key is, like it is everywhere else, to keep your wits about you and be aware of your environment.
And to put another longstanding stereotype to rest, Harlem is one of the safest communities in Manhattan. Feel free to go to Harlem, enjoy some delicious soul food, gospel choirs or jazz, and know that you are not in anything like downtown Beirut. It's a wonderful part of the New York City experience that gets overlooked because of preconceived notions people have before they actually get there.
Well, at least you spared us for a week. 
