There's a story of a guy who wanted to sell cars online. But what a huge system to write, and he didn't know if it would work or not. So he made a website with forms etc, but manualated the entire back-end by hand. The backend was a mock up. This have him the feedback that the business could work.

But this approach needs ideas that people can recognize that they want. There's the story of the early xerography (xerox) process being sold to IBM - but they carefully assessed the need for it and declined:

http://ecotopia.com/webpress/futures.htm