Anyone else feel free to correct me if I'm wrong, but if you have a car - there isn't a single place in the city I would not go to during the day. Expecially if I was with someone else. Going solo on foot may be uncomfortable if you don't know the area and are getting stares from others, but I've yet to have a problem as a mid-20's male in my 8 or so years here. Of course I wouldnt have any valuables in many sections philly.
For an idea of violence levels, see
The Philadelphia Inquirer - but don't let that map frighten you more than it should - it is for the entire year of 07 and mostly includes purpose-driven murders, not that that makes it OK.
To be perfectly honest, the most threatened I have felt in Philly has both times been right next to the convention center post-midnight when someone and their "lookout" approached me - which ended up being them wanting to sell me product.
This isn't to say not to use good sense of course.