Wired.com has an interesting wiki article describing what you can do if you are an American and get turned away at the polling place because you are not on their list:
Federal law requires states to allow voters to cast a provisional ballot if a voter claims he registered before the state's registration deadline and is eligible to vote -- that is, that the voter is a U.S. citizen, 18 years or older and is not a convicted felon (in states that don't allow felons to vote). The U.S. Commission on Civil Rights found that "countless" numbers of eligible voters were disenfranchised in the 2000 election because poll workers did not allow them to vote on provisional ballots. So it's important for voters to understand their rights and insist on them at the poll, experts say.