Are you seeing it identify itself as "Google Adsense Spider"? I haven't seen that one. I will see a google bot called "mediaSense." (or something along those lines. My access logs just rolled over and digging out my old logs are pain at the moment.) Anyhow... I'll see the mediaSense bot hit a site at the same time as someone who is not blocking google ads hits the site.

What I think is happening is that someone visits the site, activating the code for an AdSense ad. Google has its bot do a check of the words on the site to figure out the most appropriate ads to serve up based on the page's content.

That's just my assumption on what's going on.... and if we aren't talking about the same bot, then nevermind.