I just wanted to share some details on our successful setup using Openfire.
We are currently running four Openfire 3.6.4 (slightly modified) servers that are connected s2s with between 10,000 and 20,000 users logged in at any time. We are no where near capacity.
We have implemented custom plugins which send a LOT of information back to our web/application servers through the use of an XMPP gateway, and this has had little impact on the performance of the server.
So I would like to say a big THANK YOU to all the regular developers who have put so much work into developing this project. The core system is very scalable and stable. XMPP is a very complicated protocol and can be very very very unpredictable - so this is no easy feat.
We are about to begin testing on Openfire 4.7.0 and have more resources in our office, so we will be in a position to contribute more back to the community in the near future.