Here's an interesting PPC supervision service that pauses your PPC campaigns in case your web site is not responding, possibly saving a lot of needless clickthrough waste. The challenge is factoring your average down time with the cost of the service and what your ad spend is.

"InternetSupervision.com has released a patent-pending “website downtime” solution that pauses pay per click advertising campaigns at Google Adwords, Yahoo/Overture and 7Search.com when advertiser websites are unavailable and restores those same PPC advertising campaigns when the website becomes available."

Makes sense for large advertisers.