Thanks for the link. In short, it seems like only poll people they can confirm are actually registered to vote based on the fact that:

"According to US census data, just 71% of eligible Americans are registered to vote. In 2008, almost 90% of those who were registered did vote. So in any poll, it is vital to know which respondents are on the register."

But there're a few more interesting subtleties in there too, so it's worth a read.

YouGov does online polling, which allows them to get very large sample sizes for national polls. I believe they had on the order of 36,000 for their final poll. Compare that to others with just ~1000.