Developer FAQ
How can I use Eclipse with Geronimo?
First off read this on using eclipse and maven...
http://radio.weblogs.com/0112098/stories/2003/02/12/usingEclipseAndMaven.html
or read the about the eclipse plugin
http://maven.apache.org/reference/plugins/eclipse/index.html
What you can do is create a maven project in each module you work on. e.g. if you're working on the 'core' module; check that out of CVS (incubator-geronimo/modules/core) and in the core directory do maven eclipse
then refresh your project and you've a single Maven project for core with all the classpaths & source directories sorted out.
Remark: You must put the incubator-geronimo/etc directory into the %ECLIPSE-WORKSPACE%/../etc to pass this task : the project.xml of the modules depends (in relative) on the incubator-geronimo/etc/project.xml)
Unfortunately the maven eclipse plugin right now cannot handle a tree of Maven projects. So you can't checkout the whole of geronimo and do maven eclipse
in the root directory and for it to add every module & module's dependency to the same single eclipse project. Though if someone fancies having a go at hacking the maven eclipse plugin - be our guest, we welcome your patches 
One minor complication right now is that we don't have any of the modules in a maven repository. So to be able to build, say, the 'core' module you need to do a build in the root geronimo directory first - which will install all the dependent jars into your local repository. This restriction should be lifted soon - but for now check ou the whole geronimo project then do maven build
and that should install all the dependent jars in your local maven repository. Then any eclipse project for any module should have all the jars it needs (in the local maven repository).