I had signed up for an account on Ning a while back, and finally got around to trying it out. It's very easy to clone an app, and then customize it a little bit. I didn't get around to tinkering with any .php or html code, but did set up the "Who's a better runner?" page.
It's fun, and I could use it for a lot of interesting applications, especially for fun stuff that I wouldn't be able to code on my own, or wouldn't want to spend the time on it.
The Ning Blog keeps folks up to date on what's going on. There isn't much on the revenue model, though, which is going to be a key point.
Right now, they fund the development through the AdSense revenue.
I think if they offered an obvious way for folks to make a little revenue from it, you'd have more people doing development, which would really help the expansion.