After too much time (I didn't notice until 30 min's later that there is a new run.conf.bat file in \jboss-6.0.0.M1\bin in addition to the old run.conf) I got it working.
So I uncommented this line in run.conf.bat:
set "JAVA_OPTS=%JAVA_OPTS% -Xrunjdwp:transport=dt_socket,address=8787,server=y,suspend=n"
and attached in Eclipse via Remote Java Application after server startup.
Not sure how/why it works without the -Xdebug option but whatever...
You can hook it up as a JBoss 5.x server.
This is working very well so far with JBoss 5.1 setup, thanks for the suggestion.