While Matt Cutts confirmed the use of the AdSense mediapartners bot for the natural search results cache at both WebmasterWorld Pubcon and on his blog, there still had not been anything official about it on the Google website itself (although Matt is just about as official as you can get without it being on Google.com!) But with some new additions to the Google AdSense help pages to coincide with the new Site Diagnostics tab, the link has now been officially confirmed.
From the “How do your crawler and site diagnostic reports work?” support page:
The AdSense crawler is different from the Google crawler
The two crawlers are separate, but they do share a cache. We do this to avoid both crawlers requesting the same pages, thereby helping publishers conserve their bandwidth. Similarly, the Sitemaps crawler is separate.
There still doesn’t seem to be any mention of this in the regular Google webmaster guidelines, although this could be updated in the future.