The first push to get AOSP Jelly Bean of any kind began with this thread which is currently maintained by XDA Forum Member demon434. It helped give the community a place to troubleshoot issues and test preliminary builds. Now, XDA Senior Member Doomsday94 has released an unofficial preview build of CyanogenMod 10.
The build is a little shaky, hence why it is called a preview. There are a number of things working, to include:
Video (sometimes youtube crashes)
Receiving and making calls
USB mass storage
HW accelerated gui
HW accelerated video
60% project butter
And here are the things not working:
full butter experience
many other things
As you can expect, this is definitely not daily driver material just yet. However, it provides Galaxy S Plus users with a base from which to proceed and a known number of bugs to squash. It may not be long until there is a more stable version for users to flash.