There's the elector issue, but then you still have to deal with the states where electors must vote (at least once, varies by state) for the candidate that was on the ballot. So in order for a switch to work not only will the Democrat slot need to secure the magic 270, they'll have to make sure and hold onto the 270 through at least 2 rounds of electoral college voting (or blow the doors off the election so bad the people that must vote for Kerry once won't drop them below 270). Pretty hairy stuff, lots of hard backroom politicing, and on top of that she said during the 2000 recount that she was against the electoral college which could give Rove some good ammo for counter politicing in the backroom. And somehow I think Karl is a better backroom brawler than Hilary, he can use the carrot and the stick, Hillary is all stick all the time.
Now one interesting consideration: what does a Hillary 08 candidacy to for Jeb? Right now I don't think Jeb can run until 2020 if at all, the "royalty" issue (venerial transmission of power) hurt George W in 2000 and I think is still hurting him today, and would be positively brutal for Jeb. But if he's running against another former president's spouse the royalty issue should pretty much disappear.