I encourage you to read this article by “mystery pollster” Mark Blumenthal about the level of support for independent presidential candidate Ralph Nader and Libertarian presidential candidate Bob Barr. Blumenthal shows that historically, polls (especially summer polls) have overstated the level of support for third-party presidential candidates.
I still think that Barr could be a factor in a handful of states. The two I am watching most are Georgia (where Barr has high name recognition from the years he served in the U.S. House from a George district) and Nevada.
Ron Paul tied with John McCain in the GOP caucuses in Nevada, and the state has a history of relatively strong support for Libertarians (compared to the national Libertarian vote). In fact, Harry Reid would have lost his Senate seat from Nevada in 1998 if not for a Libertarian candidate who picked up a couple of percentage points.
Democrats have increased their voter registration edge in Nevada, and Barr’s candidacy could be one more thing that pushes this state toward Barack Obama.