Chosing a running mate is a huge deal. Gore totally fucked up when he picked the Jewish Leiberman. That turned off a lot of voters especially at how vocal Leiberman was about helping Israel. Even though Gore got scammed at the end I beleive that he killed his hopes with that crappy choice. It's going to be a big deal at who Obama and McCain pick. It can make or break their campaigns.
Arnold as VP would give McCain a lot more votes in my opinion