To begin with, I'll restate my position (borrowed from Pal Joey) that any polls that don't support my candidate are inherently invalid.
Having said that, any polls that have been taken that have included Nader and Barr have shown better results for Obama, as compared to polling the same group with just Obama & McCain. The thought is that those who don't have a comfort level with Obama would vote for Nader or Barr rather than McCain, given the choice.
Even more than most, polls this year will be questionable (except those that support my candidate). For better or worse, this year is different from any other, and the assumptions built into the polls are based on the demmographics of prior elections. Will Obama change the electorate appreciably in swing states with voter registration? Are folks being honest about voting for the black guy? Will this finally be the year when the young vote turns out?
"It does me no injury for my neighbour to say there are 20 gods or no god. It neither picks my pocket, nor breaks my leg."
-- Thomas Jefferson