Google Trends provides some great insight into what people are thinking about, even if they don't always help us to understand what this insight means in terms of the candidates' positioning.
For example, after the third presidential debate on October 15, there were very interesting search terms hitting Google with great popularity. Queries on "Joe the Plumber" were to be expected, but we also see the public trying to understand what is actually being said during the debate. For example, the term "repudiated" used by the moderator and both candidates, must not be widely understood if the queries for it and its definition are indicators, and certainly the pronunciation of litmus test must have been poor as "lipness test" was a very popular query as well.
Obviously Google has some insight about searches and traffic that nobody else has. It has revealed some via Blog posts and they have also made a few mash-ups available. While this is also interesting data some of the asides about query trends by location suggest it is just the tip of what they might offer.