Eric just launched the new Media Center interface for PlayOn, which he is calling vmcPlayIt. I haven’t given it a whirl yet, but I suggest you give it a try.
Update: I have successfully installed and tested vmcPlayIt, which uses PlayOn as the streaming engine/server. There are a few kinks that can be worked out, such as having to restart the program a few times. You can read all about it over on The Green Button. Overall, this is a huge first step! I’m excited that I can now stream in this new way to all of my extenders. Thank you PlayOn and vmcPlayIt!