> Any ideas on how to track that?

Two things:

1) This is probably going to have to some kind of javascript spy that
reports to a (php/asp/python/ruby) page for recording onUnload().
2) You might read the Google AdSense legalese to see if they allow it, or if
they provide it (maybe for a fee). What you want is access to their redirect
page log.

You also might look into Urchin and the ISP's that support it.

--
Jared Farrish
Intermediate Web Developer
Denton, Tx

Abraham Maslow: "If the only tool you have is a hammer, you tend to see
every problem as a nail." $$