On April 30, a poll sponsored by MSNBC and the Wall Street Journal was released. If the Democratic nominee were Barack Obama, the November results are: Obama 46%, McCain 43%, undecided 6%, other 5%. This poll didn’t list any “other” candidates. See here for more results from that same poll.
Id say that 5% is Paul voters + the 1% of third party voters.
When polls won’t list another choice, its easy to see why people think they don’t have any other choice besides the two big party candidates.