As for Palin, the obvious appeal is she is a solid conservative and a women. Nails down 2 key groups well. Being in office for not too long is a concern.
Romney is a formidable and articulate debater. He would easily best Biden, and the key reason he would be a good VP is that Romney can put his ego aside and do whats needed for the team.
I dont know about Pawlenty but dont underestimate a republican who can win in MN.
After all the bad blood between McCain and Romney during the primaries, I can’t imagine those two getting together. But then again, stranger things have happened (like Obama/Biden!)