The Spanish National Epidemiology Center conducted a study Jan 2004—Feb 2009, evaluating the use of internet databases, such as Google insights, for tracking emerging infectious diseases. They compared the Google queries regarding Influenza-like-illnesses to reported cases. Their results suggest that internet databases and tools such as HealthMap (my new favorite website!) can be used as early warning signals for an outbreak in addition to standard surveillance systems.
Although published on the US CDC website, these results should not be taken too heavily by the general public, as is often done. I would imagine the results would be strongly biased based on a number of factors.