Politico's Patrick Ottenhoff is a little premature in analyzing a Republican victory in the Virginia governor's race (Election Day isn't over yet), but his cautionary note about geographic trends is a good one:
[Republican Bob] McDonnell’s victory in Loudoun and in neighboring Prince William County will come as a surprise to many armchair pundits, who thought that all of Northern Virginia had became solidly blue. Many die-hard Democrats will blame Creigh Deeds’s lifeless campaign and the political environment.
But the truth is that Northern Virginia is often taken for granted as a powerful Democratic bloc. To be sure, Fairfax County has become solidly blue, but Loudoun and Prince William counties are more accurately full of independents who just happen to be supporting Democrats recently.
I'm fascinated by the process by which a solid Republican county becomes solidly Democratic (and vice versa), and I suspect that Northern Virginia may yet become as reliably blue as Massachusetts. But these trends almost never play out as a purely linear plotline.