Hey Paul,
Yes you want to launch JBoss from MyEclipse in debug mode, ‘how?’ you say? Well just click here:
Windows>Prefs>MyEclipse>Applcaition Servers>JBoss 3>Launch
and select “Debug Mode”
Also click the JDK node, and make sure you have configured a JDK you want JBoss to be run with. after that, you are pretty much home free. Just make sure that the JBoss server connector is enabled (root of the JBoss 3 prefs node, “Enabled”) and now you can use the server control dropdown to manipulate the server.
However, keep in mind that to use proper debugging support your servlet container that JBoss is using needs to support the JSR45 spec, which right now is Tomcat 5. I believe that the way you get this to happen is to use nightly builds of JBoss 3.2.4 that is using Tomcat 5.
A few people have mentioned that they got JBoss/Tomcat 4.1 actually debugging fine, but just as a warning it may not work for you.