How funny that I also done quite exactly the same app which came out something like 2 days after this one...
I blogged about it too : http://alexiscreuzot.com/#!/scribble/4
Congrats! Would you be willing to post anything about your success post launch? I'm an app developer as well and I love reading about other people's strategies such as this.
Good idea, I'll think about that. It's not that much of a success though (not even near Stuart's app), but I guess it's a good start for a first app without any marketizing strategy.