I would like to find something like this as well. In another post someone mentioned buddypress.com, but the developer has ceased development due to lack of time. He has released the plugin(s) to the community though. I don’t know enough php to be able to get them to a working state for me.
It is my understanding that Automattic took over the project and is working on a release soon. I too hope they hurry, I am anxious to try out a better community than MU can provide directly out-of-the-box.
I have a couple of websites I’d be willing to install BP on ASAP when it’s released! =)