This is actually advocated by the "lean" startup movement. I've read a couple of books that advocate putting "buy" buttons on content/applications that do not yet exist to gauge demand or test potential ideas. I get it, but it doesn't sit right with me.

How do you think the Brooklyn bridge got built?

I'm guessing the city built it with taxes. Probably without needing a public vote. I could be wrong though, I'm not from that area. Certainly not some guy on a virtual street corner asking for what amounts to donations.