Everyone's really making Nader's candidacy out to be something more than it is. If Hillary or Barack can't run a campaign which appeals to his voters, it's their own damn fault. Even then, I feel he'll struggle to get near 1% of the vote. People also seem to forget that Cynthia McKinney is running as a Green, and her outrageous, ultra-liberal record will appeal to the far left as much as Nader's does. The two of them will split that extreme left vote so much so as to make it completely irrelevant come election day.
I also don't think this election will be as close as it was in 2000, simply because you will be able to distinguish both party's nominees from each other, even if it simply comes down to race or gender combined with all that hope/ change/ military experience fluff which has hijacked this election.