Craig has a great post which describes how XBean can really help provide a customized XML language with Spring XML configuration extensibility. We're really happy with it so far and have moved away from XSLT and custom DOM processing to the simpler XBean model in ActiveMQ and Jencks with ServiceMix to follow suit real soon now.
Whats cool is we've just about got a working XBean Ant Task to auto-generate the configuration, the XSD and HTML documentaiton now. e.g. here's the current ServiceMix generated documentation.