Thanks lamont2718 - I think you are correct. It looks like Delta uses Gates in the B1 concourse (40, 47, etc) and Virgin uses gates in the B2 concourse (51). B1 and B2 are in the same terminal but it does look like when coming off a Delta flight one must walk in the B1 wing, walk over to the B2 wing, and walk out to the Virgin Atlantic gates - going though security on the way.