John King from CNN did a pretty solid analysis of why North Virginia (NOVA) is so critical for the outcome of the voting in the Old Dominion for President:
The one thing that King did not mention about Virginia is that not only NOVA put Obama over the top in this state in 2008: in traditionally crimson red VIRGINIA BEACH, Obama fought McCain to a draw, where McCain won by only +0.71%. Bush won that County in 2004 by +18.83%. If Virginia Beach is again very close on election night, then most likely, Obama has won the Old Dominion again.
Here is my bio of Virginia, which links to a much larger analysis from 2009. Good reading material.