I sort of remembered a site in my portfolio and went to check on it today. It's killing it in the SERPS. 1.6 million hits last month, 35k uniques. It racked up 28k referrals from google. Unfortunately, the site was built around an affiliate program that ceased to exist. I'm not making a dime off the site. Right now, I have a few links on there to send traffic to other vaguely similar sites. All that work... down the tubes. This has actually happened to me more than once. I guess the moral of the story is, be careful what you put your effort into. The bottom can drop out at any minute.