Man, this space is filling up so fast. We're working on a similar product, we're funded and a little further along (shameless plug: http://www.revisu.com). When we started working on this, there was no one in this space. Now there are a ton of offerings!

I think you hit the nail on the head with regard to what designers need from version control. We bill ourselves as Dribbble + Github for creative teams exactly for the reasons you describe.