Virginia doesn't have party registration for voters. Anyone can walk into the polling place (assuming you're registered to vote in that jurisdiction) and ask for either a democrat or republican ballot. They don't care what party you support or who you voted for in the last primary. I actually considered asking for the dem ballot so I could vote for Hillary on the theory that she is more beatable in November than Obama. But I've never voted for a democrat and I just don't think I could do it even in a primary.
That is foolish and dangerous. Either Hillary or Obama is going to be President in January, or else we'll soon be wishing one of them was (the only way I can see McCain winning is if the Democrats and LSM decide he'd be useful to them). So if you're voting in that primary, vote for whichever one of those you would be more willing to have win.