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.
What he leaves out is that this is also a good way to gauge the cost of acquisition. I've seen several products developed that never advertise on AdWords because they discovered after development that the keywords were too expensive to bid on.
"If I’d asked people what they wanted, they would have said faster horses" -- Henry Ford
This is a tactic better suited to free traffic, which makes it tough to pull off from a cold start.