Yes, he could. Really. Here's how:
If Barr can carry any states -- even one -- in a close election, he might hold "the balance of power" in the Electoral College. It's a long shot, but not an impossibility. He'll obviously play well in Georgia (15 electoral votes). Nevada (five electoral votes), too, especially with Wayne Allyn "King of Vegas" Root on the ballot as his running mate.
I'm not going to do all kinds of weird math for you here, but let's suppose that John McCain and Barack Obama come in very close -- neither with a majority of electoral votes in pocket, and Barr standing there with the difference that makes a majority.
Naturally, heavy Republican pressure will be put on Barr's electors to go "faithless" and cast their votes for McCain. If there's no majority in the Electoral College, the decision gets kicked to Congress to make. Each state's US House delegation gets one vote, and it takes 26 to win. Care to guess which "major" party controls the most House delegations?
Pressure can be exerted in more than one direction, though. And if the (still hypothetical) Barr electors stand firm, they can dictate the result: You can have Bob Barr if you go with us, or you can get Barack Obama from the Hill. John McCain is off the table.
Long shot? Yeah -- but I bet that professional handicapper Wayne Root would give it better odds than the Libertarian Party has ever had before.