A related idea is to build less of the product, eg. the backend is run manually (ie. you are the backend). I like this one better, because it seems dishonest to me to say "buy our great app!" and then not have it. But I guess you can word it as a request for interest, like the HospitalVille example.
Question: has Zyngna actually been successful with this technique? (I've only heard of Farmville).
Like I say, it's underhanded, but at least you can have a little more faith in the results.