Noam makes the case here. I don't disagree with any of his arguments, but I think the possibility of an Obama-Casey ticket is still close to zero. And that's because--it must be said--Casey seems so completely out of his depth.
Political junkies may remember that in 2006 Casey destroyed Rick Santorum in a Pennsylvania senate race noteable for its nastiness. While there may be no denying the fact that Casey's 17-point margin of victory was unprecedented for a contested campaign in a purple state, Casey's win can still be explained in large part by his name ID and the unpopularity of his opponent.
Moreover, watching the televised debates between Santorum and Casey was extremely disturbing--and not because they reminded the viewer that a bullying moralist was elected to two senate terms. Rather, I cannot think of any debate that I have ever seen at the Congressional level or higher where one of the candidates (Casey, alas) appeared so completely inexperienced and even confused. Some people found Santorum's hysterical anger inappropriate--but I actually felt bad for the Republican incumbent. Here he was--a hardworking, knowledgeable senator--facing the fight of his life against a state treasurer who rarely showed up for work and had absolutely no command of the issues.
I am willing to believe that Casey has "grown" in office, but the man who debated Rick Santorum less than two years ago is in no way qualified to be vice president. And for that reason alone, it's unlikely Obama will actually choose him.