Don't expect the Dems to be foolish enough to crossover for anyone in 2008. They want the presidency too desperately to sacrifice it.
All a Giuliani or McCain nomination would do is cost the GOP votes.
A large number of Democrats do not like Hillary and would support McCain or Guliani. I know a few, in my family, who've never supported a Republican in their lives but would vote for Rudy in a heartbeat because of the impression he made on them on 9/11 and subsequently.