Google already does this prefetch on some search results pages with the top result. For instance, in firefox if you search for "hacker news":

http://www.google.com/search?q=hacker+news

there is a link element added to prefetch news.ycombinator.com.

  <link href="http://news.ycombinator.com/" rel="prefetch">

I assume if you aren't careful in your logs, you could be overestimating your traffic.