Welcome to WebmasterWorld Guest from 188.8.131.52
Deep pocket sites sometimes get special permission to "break the rules". Who knows, maybe they hit G with a "we will do it, or not run any" and G might have caved in.
I don't think it's possible to get away with it without discovery. Google's adsense bot most likely looks for signs of not allowed code like yahoo. Would be simple for them to do it, can't imagine them not.