I've been talking about this in the Arduino irc room for a few days and thinking about various aspects of it, and I've finally put together a page about what I've named "The Unduino Challenge".
It's a not-so-serious Arduino based challenge aimed at everyone from total beginners to half-cyborgs, in which a simple objective is set - but the obvious solutions are not the most desired.
You can read more information here
Feel free to leave questions and comments either in here, or as a comment on that page (anonymous commenting should be enabled).
Hopefully there will be enough interest to give this a good go!