I like the idea of verifying human interaction... I guess that could be built into the ajax that is likely to be part of the interface anyway.
Even as I type I can see some very nice, neat, unobtrusive ways to validate that user actions have happened (an a specific page ID - without privacy issues as pages would be given random ID's not attached to anyone - and those ID's would be cheap/quick to store; on the day in memory, beyond that in a slower storage method). There will be ways to get around them (isn't there always) but it's probably enough to make it less likely people will bother. Also, it would not need to break the ability of people to link to the results (bookmark or email), you just need to store that it was a valid, human triggered, search at one point.