Matt Yglesias makes an excellent point:
The interesting thing about the 2008 election is that the political marketplace has responded to the collapse in support for Bush and the GOP in a pretty efficient manner--with the Republicans nominating someone who's somewhat less conservative than Bush and whose association with the GOP brand is relatively weak, and the Democrats running on a more liberal agenda than they've had in recent cycles.
You could make a similar observation about the institutional design of Congress and the 2006 election. There was a huge wave of discontent with the ruling party. The body designed to be most in tune with popular opinion (the House of Representatives) flipped to the opposition by a wide margin. The body designed to be slightly more insulated from momentary shifts in public sentiment (the Senate) flipped just barely, and perhaps thanks only to one ill-chosen word from George Allen. As dissatisfaction with the GOP has persisted, the party's prospects have become even bleaker, and now it stands a pretty good chance of losing the presidency too. (And if it doesn't, it will be because the two parties responded to the situation rationally in the manner Matt describes.)
All in all, it's a fairly elegant, and yet not impetuous, means of transferring power as public opinion changes. It makes you appreciate the wisdom of the system the Framers devised--something to keep in mind next time someone tells you the U.S. would be better off with parliamentary-style government.