Clinton, and you're no Hillary Clinton!" I haven't read all the diaries and comments, so maybe this isn't a very novel idea...
And it's a little cheesy and probably unimaginative, but it would be a soundbite that could step on McCain's cynical appeal to women. Palin opens herself up to it by invoking Hillary too.
I imagine it would even be more effective and authoritative than Lloyd Bentsen's immortal line to Dan Quayle:
"Senator, I served with Jack Kennedy: I knew Jack Kennedy; Jack Kennedy was a friend of mine. Senator, you're no Jack Kennedy."
Plus, the media would dredge up the old Bentsen-Quayle footage, offering a nice visual counterpoint to confirm this Palin pick as a dud.
If Hillary can't do it, I'm sure you can find plenty of surrogates to deliver it. Otherwise, I'd say that Obama and maybe even the inner circle of the campaign should just steer clear of Palin: He's running for president and there's no need to take on or compare him to a VP pick.
Any other suggestions--there are probably some much better comeback we can think up!