A WebmasterWorld thread reports that notifications are being sent to advertisers that on January 25, 2007, they will be automatically included in the Microsoft content network. Other members confirmed the email, but I do not have a copy myself.
One member said:
The email stated that my current settings will be changed and I will HAVE to start using the new Content Beta. With NO option to opt out, just stating that it is going to happen on 1/25/07.
I suspect there must be a way to opt out, but maybe it is not clear. He said he called adCenter support, but they "tried to show [him] how to opt out, but had no clue themselves."
Forum discussion at WebmasterWorld.