Joran's support for property files (through PropertyAction) is useful, but it only reads filesystem files. It would be useful to be able to read classpath resources, that way the logback.xml file as well as its properties files would be located in the same place (in the class path).

Furthermore, it would be useful for both resource and file lookups to perform system property resolution in the included file names.

For instance:

<substitutionProperty resource="config/logback/logback-${CONFIG_MODE}.properties"/>

where CONFIG_MODE is a system property. In our environment this is set to something like DEV or QA or PROD, which would allow use of substitution properties that change based on the environment.