As a libertarian/conservative I would hate the idea of a healthy and able Scalia being replaced by Obama, a liberal would balk at an able Ginsburg replaced in 2006 by Bush (when she first got Cancer).
Here is something I just made up:
What if everybody on the supreme court was replaced when a president got elected by both the new president and the highest ranking member of the other party. Party that isn't president gets 1 more pick.
Obama gets elected president in 2008. Boehner gets to choose 5 new justices. Obama gets to choose 4.
That way, there would be 1 more justice that was chosen by a Republican than a Democrat.
In theory, this would keep some of the crap presidents try to get away with unconstitutional.
When a Republican is president, there would be 1 more left-leaning justice. That could reign in the crap Republicans try to pull.
Win-win for a constitutional government.