Republicans Outnumber Democrats Online

I've seen the results of this study posted several places now. According to a Nielson report, Republicans outnumber Democrats online. From the press release:

Nielsen//NetRatings (NASDAQ: NTRT), a global leader in Internet media and market research, announced today that 36.6 percent of U.S. adults online are Republicans, 30.8 percent are Democrats and 17.3 percent are Independents. With campaign Web sites becoming increasingly important to reaching the electorate, candidates need to keep their fingers on the political pulse of the Internet.

This difference, while interesting in its own right, is more interesting when you consider the party breakdown for most polls. Pollsters almost always assume a numbers advantage to Democrats when polling. Is there really such a difference between online and total demographics? Or is this one more indication that pollsters use questionable statistical methods to bias polling results?