Note that Woodies in the above answer is a gay bar. The trendiest places are the bars and restaurants in Old City and Center City in general. The trendiest neighborhoods to live in are (in decreasing order of yuppie-ness) Center City, the Art Museum/Fairmount, Northern Liberties, and parts of South Philly close to Center City like Bella Vista.
I would check out Woodies ( ---ignore the retarded name) on 13th Street. There s typically a ton of people in there, always dancing and music. Great crowds too. From there you could always try Dock Street Pub on 50th and Baltimore. Plenty of Penn students up that way, and UPenn students are ALL yuppy-ish. Worst case? You could always check out Finnigan s Wake on 3rd and Spring Garden. It s not too far from the Fishtown Tavern either, on Frankfort (and Thompson maybe?). Fishtown took what Northern Liberties couldn t handle in terms of yuppies (little more hipster-ish in NL) and now congregates them at the Tavern. There s typically a DJ going in there, and you can t beat the bartenders and drink-girls in there...if you can get past the resident hotties!