I'm a 2nd year Engineering PhD student at Pitt. My first year I lived in Squirrel Hill, and while I know several people who biked, I tried and, well, the hills were more than I bargained for, at least with the used bike I got from Craigslist! Maybe a nice road bike would be better. Anyways, you can definitely bike most of the year, I would just say that you need to have some experience biking (unlike me!). On the other hand, I was surprised what someone said about the bus in Pgh. I feel that the public bus service is excellent, it's one of the best in the country from my understanding. Pitt (and I believe CMU) students can ride the bus, day or night, for free with their student IDs. This has come in SO handy for me, not only for daily commute but especially for off-hour transit. You can get to wherever you need riding the bus, and there are buses that go to places like the Waterfront, etc. that offer grocery stores and other things. If you're up for a longer ride, there's plenty of buses that head to the suburbs, right to some large malls (Monroeville, Ross Park, among others), if you're need to do some shopping. There's also some nice parks here, Schenley and (my favorite, used to live across the street from) Frick Park. Love the name
In my opinion, Squirrel Hill is VERY safe and quiet, but rent prices seem a bit higher there than some of the surrounding areas - you can get a pretty good feel for this just by scanning Craigslist though. Shadyside seems to be a bit more popular in terms of "the cool place to live" for grad students, but Squirrel Hill suited me just fine. Lots of students there.
In general, I would stay away from living in Oakland, lots of undergrads live there, lots of partying, can be somewhat trashy, etc. Oh, and watch out for the rental company with the pink sign "Los Lobos". Their spanish name suits them, they definitely act like wolves! I have heard good things from Walnut Capital - very professional, albeit pricey. There's also tons of local landlords with just a property or two. If you have any questions, feel free to ask.