I would go so far as to say they must have proof that they are not registered to vote in their home town/state. I think there is a lot of double voting going on, for instance here in Florida where many maintain 2 homes. It is my understanding that nothing is routinely done to make sure people are not registered in more than one place.
susie
Obama wants transient backers to vote in key states (Posted on Friday, September 19, 2008)
Barack Obama's campaign website is recommending that voters remain registered in battleground states even though they may not be living there on Nov. 4. The Republican National Committee (RNC) is raising questions about how Sen. Obama’s (Ill.) campaign is urging its supporters to vote in purple states despite living in red or blue states this fall. An RNC official said it is unclear whether that practice is illegal but argued that it is “out of compliance with the spirit of the law.” However, Columbia University law professor and election law expert Nathaniel Persily stated that “there is no problem” with what Obama is doing. He noted that thousands of voters will be eligible this year to vote in one of two constituencies.