One more idea to add to the mix:

* Before even *thinking* about starting development, make a landing page to collect email addresses for people who'd be interested
* Send people to these landing pages via google/facebook/forum threads
* Make a few of these landing pages
* The landing page that gets the best conversion rate (visitors vs number of emails collected) is the app that gets built!

Thanks for reading! And if you want to get in touch, I'd love to hear from you: chris.hulbert at gmail.