Ask HN: Ideas for mobile marathon?

My plan is to build one same mobile application for iphone, android, windows phone 7, blackberry, (nokia) and (bada) and blog my experience with each of the platforms.

Any suggestions on what I should build?

One of my ideas was to build a mockup viewer for balsamiq.

I will be spending around next 1.5 months on this. Only constraint is it should not be a 3d game. Anything else is fine :)

For each of the platforms I will be using the tools and sdks for that platform ie no cross platform frameworks. I will periodically blog experiences with each platform and release the code under MIT license.

So let the brainstorming begun :)

p.s. If any startup wants to utilize my marathon to get their mobile app built, I would be more than happy to do it for free as long they are fine with taking over the project once I have released the v1