Internet Polls

• Is there an inherent bias in online polling data?
• Are certain groups more likely to respond than others?
• Is there such a thing as a rampant Liberal?
• Zogby seemed always to underestimate Obama's polling
• What about Fox/Sky News Polls/ Metro Polls

Nice article in Scientific American that shows how right and left are defined by a few characteristics.
Can the bias be shown to be systematic? If so can it be corrected?