Atrios notes that "With Landrieu winning, the Chafee/McCain double-switch fantasy enters the realm of (unlikely) possibility."
Well, first, I'm glad that Ms. Landrieu won; playing the Louisiana rules to try to eke out a win in the runoff by fielding tons of candidates may have been legal, but I'm still not overly fond of it.
Second, if that double-switch happens, a lot of people are no doubt going to be whining and crying about it. Honestly, it's just not that surprising; moderates on both sides are squeezed together due to the burgeoning polarity of the political elites in the United States, and switches like this may become commonplace. Indeed, if party discipline is truly on the rise (and by all accounts, it is) then this makes sense as well. It's a way for moderates or cross-partisans (leftist Republicans and rightist Democrats) to gain notoriety and power within an environment where they aren't going to be powerful in any case, and where (like in the case of Jim Jeffords) they stand the chance of being frozen out almost entirely. It's also a way for senators to leverage the length of their terms; in the house it'd be rougher, as it might provide an "in" by which a challenger could take your seat.
What the real question is is whether the United States will start seeing party discipline on the level of, say, a parliamentary system. It's quite possible, actually, as the individualistic nature of the system gives way to this sort of ideological polarization.