Hi,
Just committed the files to the tomcat module that allow you to see Catalina process running inside Apache Geronimo. It's not at all functional as Catalina isn't able to find its configuration file, but worth to try out the org/apache/geronimo/Tomcat configuration.
See http://wiki.apache.org/geronimo/Tomcat for some additional information.
Once built, run Tomcat via the following command:
$ java -jar bin/server.jar org/apache/geronimo/Tomcat
...
02:37:48,934 INFO [ConfigurationManagerImpl] Loaded Configuration geronimo.config:name="org/apache/geronimo/Tomcat"
02:37:49,094 INFO [Configuration] Started configuration org/apache/geronimo/Tomcat
Can't load server.xml
02:37:49,465 INFO [Catalina] Server startup in 0 ms
02:37:49,465 INFO [Daemon] Server startup completed
Have fun!
Dobranoc, Jacek