I found this Time Magazine article interesting with respect to all those who have proclaimed they would vote McCain or not at all if there candidate is not nominated:
It is true that between 25 and 45 percent of Democrats who voted for Clinton have told pollsters that they would vote for John McCain in the fall rather than Obama, but is that threat credible?
There is considerable evidence that some of those voters are letting their emotions do the talking—and following in a long tradition by doing so. According to ABC News' polling unit, in the spring 1992, only 63% of Democrats who voted for someone other than Bill Clinton in the primaries that year said they would vote for Clinton over George H. W. Bush that fall. In 1996, 66% of Republicans who voted for someone other than Bob Dole in the GOP primary said they would support Dole against Clinton that fall. Al Gore suffered the same apparent dropout problem; only 64% of Democrats who voted for his rivals during the primary said they would be there for Gore in the fall.
The full article:
http://www.time.com/time/politics/article/0,8599,1811112,00.html?cnn=yes