I wonder how much the in-house expertise of developing Chrome is contributing to the GoogleBot analysis stuff. Having people who aren't afraid to go knee deep in WebKit and produce some nice fast library for partially evaluating layouts, etc, is probably quite a nice thing.

As one of the sibling comments here mentions, ad detection is a pretty well-solved problem by now, through size, position and origin matches. The interesting part here I think is how they're actually doing the 'virtual rendering' (and presumably, reflowing it for each of their mentioned typical browser resolutions) to check what's actually 'above the fold'.