Try and add /lib/tools.jar from the J2SE SDK to JBoss' classpath, I seem to remember this fixing the problem.
JBoss typically finds the tools.jar through the JAVA_HOME environment var, maybe you haven't set this yet?
JAVA_HOME is set to C:\j2sdk1.4.1_01\ and lib\tools.jar exists there. I have also tried to add tools.jar manually to the classpath, but the situation doesn't change.
Any other ideas?
ok, problem solved: Eclipse was pointing to an other Java-dir than I expected. Now it works (for the moment :-)