So, it's finally upon us.
On the GOP side, everybody's looking to California, of course. If Romney can win the statewide popular vote (the polls are all over the place), he's back in the race, at least temporarily. California awards its delegates per Congressional district, with three delegates per district (regardless of the number of Republicans in the them), winner-take-all. With that setup, the delegate total for the state may wind up closely split between McCain and Romney, though I think the statewide vote total is still much more important in determining post-Super Tuesday momentum.
Two somewhat-overlooked but quite important states are Georgia and Missouri. Both were once expected to go for either Romney or Huckabee, but recent polls in both have shown McCain tied or slightly ahead. If McCain can win the popular votes in Georgia, Missouri, and California, along with the big states where polls have shown him strongly ahead (New York, New Jersey, Illinois, etc.) the Republican contest will be effectively over.
As for the Democrats, with the polls so close in so many states, well, it looks like we're in for a very fun night.
The polls in the first major state, Georgia, close at 7:00pm.