Assumptions: Target is JBossAS05_Beta2
Prerequisite:
a) Download jboss-5.0.0.Beta2-src.tar.gz, since you need third party libs.
b) Have JBossIDE 2.0 installed.

Now start:
1.From Eclipse menu choose New>Project>Java Project>Create Project From Existing Source. Here you have to use ?thirdparty? as project name, case sensitive. Browse to thirdparty directory under the unpacked jboss-5.0.0.Beta2 parent directory and choose it for the existing source. Finish it, and you will get a project named thirdparty.
2.Create an new workspace for eclipse. Then ln -s ?path to jboss-5.0.0.Beta2 thirdparty directory?. Then you' ll get an link named ?thirdparty?to the directory where all dependencies lived.
3.Add a new SVN repository ?http://anonsvn.jboss.org/repos/jbossas/tags/JBoss_5_0_0_Beta2/? to eclipse. (SVN plugin for eclipse can be found through the old ?HowToBuildJBoss? topic on jboss wiki). Pull out all sub-modules you need to examine. Check the project module dependencies, and pull out all those depended modules too.
4.Change to those newly pulled out projects under the workspace. And build them using command line ant separately. Pay attention to project dependencies showed inside eclipse Project>Properties>Java Build Path>Project page. Depended project should be build first. Finally refresh projects inside eclipse, and create a new project set, including all your pulled out project. Then build the project set.
5.Now you can play with your source code, and enjoys eclipse's short cut key!

You also need to pull out build and tools module, they are needed for compilation