The Michigan Messenger published an article today looking at how third parties receive access to ballot access in Michigan. The article says:

"For the minor parties to appear on the ballot for the Nov. 4, 2008, general election, the top-of-the-ticket candidate - whether it's a presidential candidate or a hopeful for a statewide office such as a university trustee position - must receive at least 1 percent of the vote the successful Secretary of State candidate received in the Nov. 7, 2006, general election. The parties qualified for this November are the Green Party of Michigan, the U.S. Taxpayers Party of Michigan, the Libertarian Party of Michigan and the Natural Law Party. That only applies to Michigan, and even though the Green Party, for example, may have a strong presidential candidate, the candidate may not be on the ballot in a more conservative state."

...

Those parties that did not meet the necessary threshold in November 2006 can still qualify to be on the general election ballot with a petition drive. They must first file as a new political party even if they have been around for many years. The party must collect petition signatures equal to 1 percent of the votes cast for the governor. That comes out to 38,013 signatures, and the deadline for turning in petitions is 4 p.m. July 17, 2008."

As with other aspects of the United States electoral system--such as the presidential debates--ballot access is another example of how structural hurdles limit third parties' ability to be elected.