Replicating 'Build a startup in 7 days' challenge?

I originally saw the 'Build a startup in 7 day challenge' "http://autocarlog.com" on HN earlier today and got hooked on the idea. It just seemed to happen, I came across a project I'm excited about and can be done in 7 days. The project is actually based on John Nunemaker's post, which I mistook. It was about Projet Euler, a quiz site about mathematical questions which requires the help of computer to solve. My project will be similar except will feature programming questions. It isn't like Stackoverflow, which is more of an answering service. I figure every week I'll post a question and anyone with an account can answer it. What do you guys think? It will be written in Rails and maybe Haml. I'll open source it if you guys want. I'm more interested in if this project will be viable and if it has already been done (I can't find any.) This project is meant to challenge amateur/expert programmer whatever the language.