I am fairly certain Obama will win Virginia. The growth in No-VA over the last 4 years is mostly in the demographic groups favorable to Obama. This is where the ground game comes into play and it should add a couple points to his margin.
The growth in northern Virgina isn't a panacea for dems/Obama. (I realize the 'party of government employes against America' has an advantage with gov employees) Many/most of the people are from the midwest and will just as easily vote for a centrist Republican as a leftist democrat. They are different than the rest of Virginia, but they aren't Vermonters or anything.