Talk about a sleazy host. While checking a few things around my website, I accidentally mistyped a URL and got a 404 page. The 404 page had AdSense on it!!! Not my account, but the web host's. Naturally, I implemented a custom 404 through htaccess that would stop the host's default 404 page. So webmasters... beware. Check what is being displayed on your 404 pages. If there is AdSense, your host is a douchebag. Coincidentally, the website I own on this host doesn't rank didley. And... don't host all your sites at the same place!
you can read this thread http://forums.digitalpoint.com/showthread.php?t=104815&highlight=404+pages+adsense for more information
Whoa. I had no idea I *could* put AdSense on my error pages - but it doesn't feel right, so I *won't*. I don't understand why they'd allow it, especially since their TOS say that they don't. I am not taking any chances.