Check their website - the poll is not conducted per se, you click on a link. So the population is already self-identifying.
Thanks for the update. I guess the main usefulness is that they do it on a continuing basis, so there is some validity based on comparisons from week to week to week among a certain population — even if it may not be something you can take as a universal poll.
From a Cain perspective, the fact that caucus goers in Iowa like him is something he will no doubt try and leverage into other support — as he should.