See related discussion from 3 years ago: http://news.ycombinator.com/item?id=1148200

And as I said then:

I think the intentions behind the idea - trying to do some societal good with your web site - are great. It's just seems odd to me that the way you handle not being able to find what the user wants is by showing them something unrelated that is guaranteed to be not what they were looking for. What is the attraction to doing this on a 404 page, vs say a banner ad (as suggested elsewhere in the thread)?