There's a limited amount of connectivity over RDU to/from LHR. I doubt its enough to matter, but it's there. Truth told, I don't remember a flight from DCA to RDU that an announcement hasn't been made regarding the gate to London for connecting passengers. And on my last return from RDU to DCA, there was a gentleman beside me who had flown in from LHR.

There is quite a bit of corporate traffic on that flight. Glaxo is the biggest, which I've already seen mentioned. Quite a few international businesses are located adjacent to RDU in the Research Triangle Park. I might be wrong, but I think that may play at least some role in why RDU has almost the same amount of O&D traffic as CLT from a much smaller population base....along with the presence of WN which gins up some traffic on its own.