Of course he is. He's an opportunist, and he always has been.
But after the GOP gets done excoriating him, they would do well to note that Specter's self-interested calculations were undoubtedly correct. He couldn't win the Pennsylvania Republican primary, but no one who could win the Pennsylvania Republican primary could win the general election in Pennsylvania. If the Republicans want to find a "real" traitor they should blame Pat Toomey, who never had any realistic chance to become Senator, but did have what turns out to have been a great opportunity to turn Pennsylvania blue.
The GOP's loss of Specter's seat is just another step in a decades long political realignment. First the GOP captured the South, and now the South is capturing the GOP. Good luck with that.