I know there are already plenty of GAP/C&O Canal trip reports, but I also notice a steady stream of questions about this tour. I'll throw my report into the mix as well in case it's helpful to someone.

http://tourdetech.posterous.com/

This was a credit card tour, so I included reviews of the places I stayed/ate along the way. I started at Station Square in downtown Pittsburgh (more ceremonial than anything) and then got a ride to Homestead to pick up the latest extension of the GAP (trailhead is not all that obvious - see my notes for the address).

Cheers.