This sounds like it's going to be really cool.
AppleInsider has an article about 10.4 Server features. One of the new features is an "iChat Server" so you can run your own private and secure iChat server. The best part? It's based on the open source Jabber Server.
The iChat server works with both the iChat client in Mac OS X Tiger and most other popular open source clients available for Windows, Linux and even PDAs.
Apple's press release.