I wrote an article about finding out whether prospective customers will buy your product over at Boxes and Arrows. You should read it!
"When we’re building products for people, designers often do something called “needs finding” which translates roughly into “looking for problems in users’ lives that we can solve.” But there’s a problem with this. It’s a widely held belief that, if a company can find a problem that is bad enough, people will buy a product that solves it.
That’s often true. But sometimes it isn’t. And when it isn’t true, that’s when really well designed, well intentioned products can fail to find a market."