I also wonder if Ken Cuccinelli has his eye on that seat, now. I have heard McDonnell say that he isn't interested in the VP slot, and he will absolutely complete his term as governor, and he hadn't expressed any interest in challenging Webb. But, as you elude to, there could be some pressure brought to bear on him.
With respect to Webb, to me, he seemed disinterested in being Senator, almost from the day he was sworn in. Other than the Iraq war and his visible dislike hatred for George Bush, I haven't heard him discuss anything with any real passion, really - with the possible exception to GI Bill improvements he sponsored his first year in office.
Webb also seemed completely detached from voters; he held NO townhalls during the 0bamaCare dustup, very rarely went to glad-hand at public events and it was damned near impossible to get through to his offices. He also strikes me as someone who has the personality of a door knob.