I'm sorry I didn't catch this thread earlier. A friend of mine and I did Philly --> Reston, VA back in April (we went by way of Baltimore and DC). Last week we rode from Philly --> NYC. I did both fixed.
On the Philly --> Reston trip, we took Baltimore Ave. in Philly to Route 1 and then continued on Route 1 all of the way down to Baltimore. Route 1 was great, however, it is questionably legal for bikes. We didn't have any trouble, though (cops would just pass by without hastling us). Route 1 turns into a big 4-lane divided highway, but the pavement is really nice and there's a huge shoulder (big enough for us both to ride side-by-side). The only questionable spot is a dam right after crossing the Maryland border (it's one lane each way with no shoulder).
There are tons of rolling, steep hills around the Maryland border. Make sure you at least have a front brake. I normally ride brakeless in Philly, but I was really glad I had one on the downhills (if you're not careful you can reach speeds in excess of 100km/h with a heavy, loaded touring rig).
There's literally nothing between Kenett Square, PA and the suburbs of Baltimore. We had to make an 8 mile detour just to get lunch. Work under the assumption that you may be stranded without water for up to 3 hours of riding at a time. When between Kenett Square and White Marsh, Maryland, make sure you take every oportunity to buy water that comes your way, because they're few and far between.
I wouldn't recommend riding past White Marsh, Maryland after dark. This is coming from a guy that lives in West Philly.
On our Philly --> NYC trip, we took route 13 (Frankford Ave.) to Old Route 1 (Lincoln Highway) to route 206 to route 27 to Newark to the PATH train into NYC. That route was really great except for route 27. As route 27 gets closer to Newark it becomes a four lane road with no shoulder. The cars were really flying and it wasn't very safe to be on the road in most areas. Where there were sidewalks in some parts, they were really crappy and didn't have ramps at intersections (jumping curbs at every intersection is annoying on a touring setup).
Here's my account of the Philly --> Reston trip:
http://www.sultanik.com/blog/RideToReston
When are you planning on being in Philly? Let me know if you'd like me to organize an "escort" into the city. Best of luck!