G have stated publically in both forums and conference Q&A sessions that they DO collect clickthrough data
occasionally, although they swear blind that they don't use it yet.
It pops up on certain terms more often (SEO related terms are quite common), and just because you can see it, someone else searching for the same term off a different IP may not, as its purely random. IIRC, they said its runs at about 0.5% of all queries during those periods when they run it.
Most people of course wouldn't notice it anyway, which is why you don't hear a lot about it