Stephen Heise (via SearchEngineLand) identifies some advertisers that are cloaking their AdWords URLs:
spammers create ads that display your URL but redirect to a spam site of their choosing, cloaking the redirect so Google can’t tell what’s going on.
This is actually a close relative of Referrer Laundering and uses many of the same techniques. In fact, referrer laundering wouldn't be possible if advertisers weren't able to fool the AdWords bot using the cloaking technique that Stephen mentions.
The techniques that these 'sploakers' use are reasonably sophisticated and frankly kind of hard for Google to detect via bots. They utilize server side code to examine the request and determine if the ad is from a Google IP or contains a Google referrer. If the IP is Google's, the website serves a different page.
Unless Google intends to hide/mask their bots (which portend all sorts of issues), they will be reasonably easily identified.
Google has (at least) two weapons against these techniques: their toolbar and the wealth of information it provides about user activity (you do have PageRank enabled don't you?) and their web accelerator proxy. They don't need to find every ad - they just need to identify advertiser accounts (which tend to have lots of ads) that are using deceptive techniques.