Arlen Specter (Was-R-Now-D, Penn.) switched over to the Democratic party today, CNN has the story here. CNN reports that he's the 13th Senator to do so since the direct election of Senators in 1913.
Is there any pattern to these decisions? The Senate reports here on the 20 members who have switched parties since 1890. What's striking from a quick look at that list is the durability that these Senators had after switching parties. More often than not, they won another election after the switch. It speaks against the strict bipartisan model, assuming the underlying constituency doesn't change (perhaps that's incorrect to assume?), though it could also speak towards everyone moving towards the center as well.
There's a dissertation's worth of ideas to take from looking at this phenomenon...on to the idea list they go!