If it's presented as "We're building this product - tell us what you want it for" it would tend to select the people who are really keen. That's the kind you want to start with - but you're right, it is a subset, probably a very small one. So maybe this is better for operationally getting started, rather than measuring market size.

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).