Publishers running AdSense on their pages may find that the Mediapartners-Google bot – the special Google bot used by AdSense to determine ad targeting on a publisher page – is actually sharing the results of those crawls with the main Google search database.
Greg Boser spotted it when pages being served strictly to AdSense began showing up in the main search database. And cache dates and times are matching exactly with when the Mediapartners-Google bot visited the page for ad targeting purposes.
How significant is this? At this point, it is uncertain, although Google clearly states that being an AdSense publisher does not help with search engine rankings. And it seems that the Mediapartners-Google bot is not adding new pages to the search index, but rather updating pages currently in the index.
Postscript: Matt Cutts has confirmed that the AdSense mediapartners bot is indexing for the main search index.