Just saw a huge Circuit City ad on this About.com page. I clicked on it by mistake, and here's what I get:

"We are currently performing scheduled system maintenance Please try again in a few minutes."

Here's my proposal: have a simple web service that says whether your site is up or not, and require advertising servers (here, the ad is served by DoubleClick) to ping it before they serve the ad. Of course you don't do it for every single impression, you do it server-to-server every minute. This way, you don't buy useless ads that ruin your reputation instead of bringing new business. Very simple things such as a web service that says yes or no could improve the web for companies and for users.