@lunchbox1175 are you sure about your info? It has been my experience that they do not wait, even if you are on the same airline. I agree that if you are flying the same airline, and the gates are reasonably close, 20 minutes would probably be enough. But, if you happen to be on a regional jet, or one of your flights is the first leg of an international route, you could end up on terminals far apart (think LAX, ATL or some of the other large US airports).
I've not flown in/out of Philly, so I really can't comment on that airport.
Personally, I would check each airport for their recommended minimum connection times and stick by that and not take any chances.