I'm not all that surprised.
Arlen Specter has never struck me as a particularly party-line kinda guy. He was a pro-choice Republican when Republicans routinely made anti-choice a national party platform plank. One year when Specter was up for re-election, pro-choice women in Pennsylvania (who'd normally swing liberal, hence Democratic) voted for him in droves.
He's always struck me as a bit of a flake, too.
But now he's a Democrat. So there you go.
Specter is losing to a right-wing-backed candidate in the Republican primary, Pat Toomey, so his party switch is clearly tied to his political ambitions. But as much as the Republicans are exaggerating this claim, Specter's own claim that the Republicans have moved too far to the right is also suspect.
What's really happened is that the Democrats have moved so far to the right that the party platform of the Republicans, circa 1984, now looks like the Democrats'. The Republicans may have left Specter behind, but the Democrats have moved right into his lane.
Which means that there's almost no discernible ideological divide between the parties. They vie for power, party power, and really, nothing else.
Coke or Pepsi?