Github for product design

A short time ago, there was a lively discussion here about getting designers to contribute to open source projects: http://news.ycombinator.com/item?id=2267861 Yesterday, we saw an example of an independent contributor "mocking up" his ideal notification interface for the iPhone: http://news.ycombinator.com/item?id=2298585 Question: Is there a tool that would make it easy for multiple folks to collaboratively design stuff and iterate on a design? Github for product design? Multiple designers suggesting things for one feature, and some way to collaboratively edit stuff, with history on the decision making process? As a "product guy" myself, I am constantly designing stuff using Balsamiq mockups. I'm eager to use the forthcoming myBalsamiq, which seems to have some group / collaboration features. But I think I am suggesting something far beyond what Balsamiq is working on.