I've also never liked those coming soon pages. They usually give no information, only a vague promise and signup box. Example of that is LaunchRock. My sense it doesn't work, if anyone has any statistics ... I'd guess that less than 2-3% subscribed share their link to get to the priority list.

My suggestion for new startups is to put an information-rich page with screenshots and explanations how that idea came to life, how it will change your life, etc - of course if your goal is to get people interested and to signup :)