My weekend project: alarm clock that makes you prove you are really awake

I've been reading a bit about sleep cycles, timing, and trying to be more awake lately, and I had an idea - an alarm clock that makes you do a simple math problem before it will turn off. That way, you wouldn't turn off your alarm while still half asleep, without consciously realizing it.

A few hours with heroku and twilio later, I have this: http://www.getwakeupcall.com

I kind of like it, because it seems simple and useful. But if I do pursue it, I'm not really sure how to. A subscription web service is easiest and simplest for me, but if I go that route I would have to charge much more than I would for, say, an iPhone app with similar functionality (since the web app calls the user's phone, potentially many times per day, and a local app would probably not have so many ongoing costs).

I'd love to hear your thoughts on 1) whether this idea is worth pursuing and 2) if so, in what format?

Many thanks!