Here is an article on the validity of Internet polling. Many researchers do surveys on the Internet because it is cheap and convenient. This article points out the problems in making sure the research sample is representative of the population being studies.
About Online Surveys, Traditional Pollsters Are: (C) Somewhat Disappointed
To be sure, traditional pollsters and political consultants do not view online surveys as scientifically acceptable. The sampling of consumer opinion tapped through an online questionnaire cannot be as random as traditional measures, they argue, and it is easier for people to misrepresent themselves and their opinions online. Another critical difference is that in traditional polls, participation is voluntary, not paid.
“Voter polling by the Internet is not yet viable,” said Joel Benenson, a Democratic pollster working for the presidential campaign of Senator Barack Obama of Illinois. “There are some uses for online polling, but it still misses out on too much of the population for us.”
Despite the strong skepticism, Internet-based survey results are likely to get some publicity during the 2008 elections, and executives from companies that conduct these surveys hope that they can use the attention to gain credibility for their methods.