BYOBs in Center City
Matyson was awesome. I did a post a few weeks ago on them and Caffe Casta Diva. http://www.chowhound.com/topics/463827.
I also found the following website/blog: http://marksquires.com/byob.htm that lists seemingly every byob in the city, with some of the author's own thoughts.
We just had an excellent dinner at Mercato, on Spruce between 12th and 13th. They take early reservations - I think up to 6:30. Cash only. Noisy, very popular.
If you don't mind walking to 17th St., you can't do better than Branzino, on 17th just north of Spruce. Check if it's open Mondays. Our favorite byob in that area.