In the Agora provides some evidence that traditional polling leaves out younger voters (who are likely Paul supporters):
This Pew Research Center survey is perhaps the most comprehensive to date in working to determine what impact this has on polling data. The Pew survey found that cell-phone-only Americans are disproportionately younger, less affluent, less likely to be married or to own their home, and more liberal on many political questions. Yet “the absence of this group from traditional telephone surveys has only a minimal impact on the results.”
Notably, the Pew survey was conducted over a year ago, and the percentage of those now cell-phone-only has nearly doubled in that time. Perhaps a new study using the same techniques would reveal a different conclusion. Either way, though, “minimal impact” is in fact significant in the context of primary polling, where even a couple percentage points can make or break a candidate’s future.
Although Rep. Ron Paul is essentially libertarian, many of his positions - opposition to the Iraq war, greater civil liberties, etc. - would traditionally be defined as “liberal” in the lexicon of some members of the press. And voters (particularly younger ones) who align with those positions would seem to be the very people Ron Paul counts among his most ardent supporters.