Jean Edward Smith reminds us that the nine-member Supreme Court is not a creature of the Constitution, but of Congress. The number has varied from five to 10 (for two periods, it has been an even number, which seems passing strange for a court of last resort--as Justice Jackson put it, "We are not final because we are infallible; we are infallible because we are final," but dissents were rare in the first hundred years of the Court's existence).
So, should a Democrat be elected in 2008,and should the Democrats control the Senate sufficiently to beat back a filibuster (a tall order perhaps), there would be nothing to stop them from adding, say, a couple of justices to bring the court's membership to eleven. Assuming a couple of reliably "liberal" appointees, and the current conservative majority might disappear.
Is this likely? No. What is more probable is that the Court (calling Justice Kennedy...) will prove out Finley Peter Dunne's maxim, "whither th' Constitution follows the flag or not, th' Supreme Court follows the illiction returns."