Let's say you get 20 people to signup, all of whom think it is (rightfully) vaporware. Well, at least now you can be sure there's a real market (>>20) to justify development. Even if you drop those 20 on the floor... the validation is more than worthwhile.

My experience in doing this has been that a surprising number of people who click the buy button still put their email address down afterwards, and none of the people I've talked to who did were remotely miffed about the deception.

It probably depends on the audience, but people are more understanding than you'd think about the need to look before you leap, especially when they know you're just a solo entrepreneur.