Jeff Atwood has not publicly released any details yet. However, his recent interview on Mixergy (http://mixergy.com/jeff-atwood-coding-horror-interview/) indicates that it is an open-source project which can be run by anyone on EC2. In the same interview he indicated that he was targeting a neglected software category that he considers to be broken, in the same way that online technical Q&A was broken before StackOverflow.
Relevant Mixergy interview excerpts:
"Let’s go back to the Podcast. Think about the way we started “Stack Overflow.” We identified something on the web that needed to change. We viewed it as a strong villain figure that everybody vilified and hated. If you think about the software that you use on the web and the websites that you use, there’s still a lot of software out there like this. It’s kind of a forgotten software category in my opinion. But it won’t be when we’re done, I think.
...
We’re still building it. As I talked about at the start of the show, you get tired of explaining it after certain points, it’s like “No, just go look at this thing.” So we’re building up enough of it so that I can stop explaining it and just show it to people and they’ll see what it is. But I can say it’s a fully open source thing meaning if you want it, you can just take it, run it, grab it, install it on Amazon EC2, install it on your PC at home, wherever. It’s fully 100 percent open source."