Finding accommodation in Philadelphia might take some time, particularly if you are coming for a short period of time, so be patient. Fortunately, there are plenty of housing options.
First thing you should do is to get a U.S phone number. You can easily get a pre-paid one in any AT&T office, Criquet (this one is supposed to be good), or any other provider really. Take into consideration that unlike in other countries, here caller AND receiver of calls pay for the call, so texting is more common.
Laundry machines in many houses work with coins. Loading a tank is worth normally $1, and $1 for drying. This might shock you the first time you see it (it shocked me at least), but apparently it's normal.
If you smoke, this might be a good time to quit. Every potential landlord require non-smoking individuals. In general, smoking is more frowned upon here than in Europe.
Additional links for medium-long stays
http://www.business-services.upenn.edu/offcampusservices/?g=housing-search/temporary-housing
Some Lab members have stayed in Chamounix for short periods. Most people there are students and people on holidays (at least in August). It's clean, reasonably comfortable and the surroundings are beautiful (its in the middle of the Fairmount park), however a little far away (bus #40 goes there, it takes around 40 minutes and there is a 15 minute walk from the bus stating) and you will share a room with around other 5 people.
Also, don't forget checking the options that uPenn has to offer. Normally a bit worse quality/price ratio of what a private owner might offer, but always very conveniently located and safe.
http://www.sas.upenn.edu/elp/housing_international_house
Transportation
Have a look at transportation. If you need to use the public transportation you will need either Tokens or a Pass (weekly or monthly). Tokens you can get them at certain train stations (see link) and are the recommended option if you use public transportation occasionally or, to be more precise, less than 11 times a week. Being enroll at UPenn you get a 5% discount over the regular Pass price (check it out here).
Also, there are some services free of charge for UPenn members (you just need to show your PennID) like LUCY. If you live in West Philadephia or Center City, there is a shuttle mini that can ride you home for free if you show them you Penn ID. This is specially useful at night, because it runs until 3AM.
Here's a Google Map (Caleb put it together stealing components from other people) showing major grocery store locations, SEPTA locations, and some neighborhood names. If you want your crib put on the map, or if you have advice/feedback, let Caleb know.
Here's a Philly map of farmer's markets (and food kitchens and such...)
Here's Trulia's map of crime in Philly.