This stuff is still pretty fluid, but the thinking from the most recent town halls is this:
The virtual base would be made up of lines of flying from the main base. For example, if MCO were opened as a virtual base, you would be based at JFK. They would move x amount of flying to MCO. If your seniority could hold it, you would be able to fly trips that originated and terminated in MCO. You would still be considered a JFK-based pilot.
The virtual bases mentioned were MCO, OAK, and IAD with MCO starting spring 08 and possibly becoming a permanent base in the fall.
This is all subject to change because they are still thinking about the best way to implement virtual basing, but I believe that it will happen.
Hope that helps.
BL