+1...just make sure you ship before everyone forgets about you :). I've heard 10% conversion from signup list isn't unrealistic when products finally ship.
It's great to have a blog with some traffic where you can find interested future users. If you have enough traffic, you can keep the signup-to-invite time short so your conversion will be better.
Keep in mind that if the idea is novel and something people want, you'll get most of them converted no matter what if you initially promised that novel thing during the signup.
