I have exactly same question.
I've allready get more information about. The cirrus RTMFP is just for developers, and we have a limit number of users (I think it is about 20 simultaneous conexions, but not sure about the exacltly number, I can't remember).
So for massive use of this services we have three options:
1. Buy the flash server and install a server.
2. Contract a Web Service that offer flash service (I only know aws.amazon.com)
3. Use a opensource server implementation (Cumulus or Arcusnode).
Thank you for your answer!
You can also take a look on this thread http://forums.adobe.com/message/3826392#3826392
"In Cirrus (codename) has no limits to the number of concurrent users you can have connected with your developer key, although we ask that you let us know if you'll have more than about 10,000 concurrent users
and maximum possible number of connections on AWS services is 10,000.
BTW, WebRTC is coming...