I concede that the idea of Bob Casey as a running mate has more than its share of flaws. But, if the choice is between him and Webb, Casey has some key advantages, too. Neither Casey nor Webb are especially gifted on the stump, but Webb strikes me as more prone to going off message. (On the other hand, he's obviously brighter and more knowledgeable than Casey, particularly on foreign policy.)
More importantly, Pennsylvania is just a more critical state than Virginia. Not only does it have more electoral votes (21 v. 13), but it's hard to see how a Democrat--even Obama, who can obviously change the electoral map--wins without Pennsylvania. (And, unfortunately, McCain is going to have a real shot at picking off Pennsylvania if Obama is the nominee.) On the other hand, it's easy to imagine them winning without Virginia. If you assume both men would be equally helpful in winning their respective states, which I think is a reasonable assumption, then Casey has the advantage here.
Again, I'm not saying Casey is a great pick--he has serious liabilities, and his virtues are limited. But his one clear virtue is a really, really important one.
Update: I should add that, all his trouble with working-class whites notwithstanding, Obama would probably have a better shot at winning Pennsylvania than Virginia. So if both Casey and Webb are equally helpful in winning their respective states (e.g., they make you 25 percent likelier to win), Casey would be more likely to deliver you Pennsylvania than Webb would Virginia.
Also, a couple people have asked why not Rendell if Pennsylvania's so important. The problem with Rendell is that he basically appeals to the same demographic Obama appeals to. See my previous item on the 2002 Rendell-Casey primary--Rendell's base, like Obama's, is the wine track. Now, he obviously did better among white, working-class voters than Obama is doing now, but he's not exactly beloved by that group. I don't see how having him on the ticket would offset Obama's weakness there. And that's the demographic you worry about losing to McCain.
Finally, commenter lonestarpedro makes a great point:
Regarding Casey, if Obama needs to re-energize Dem women after an ugly battle with Hillary, putting a pro-life VP candidate on the ticket is the wrong way to go.
Like I say, there are several very good reasons not to put Casey on the ticket. (Though this is also a big knock against Webb.)
Update 2: A reader writes:
Here's why Rendell would probably work at least as well as Casey in PA for Obama, and probably better: I don't think PA voters--including white, working class voters--will vote for the Obama/Rendell ticket on the basis of some class affinity for Rendell. They will vote for it out of a combination of state pride--and more crucially--the vague, but real sense that their outspoken governor will, especially during a recession, give the state real juice in Washington as the Vice President. It's a vote for "Eddie" Rendell, the fast talking guy who makes shit happen for Pennsylvanians--nice guy rookie Senator Bob Casey can't match that in a million years. Rendell and Obama will run up such a big vote in Southeastern PA alone that the they won't even have to bother counting ballots in the rest of the state.
That makes sense. On the other hand, if you're picking your running mate with an eye toward locking up PA, you'd better make sure he can deliver it. I still think Casey does slightly better by that measure, though worse on almost every other. (And I think an Obama/Rendell ticket would likely carry the state.)