Set environment variable MAVEN_OPTS to:

-Xdebug -Xnoagent -Djava.compiler=NONE -Xrunjdwp:transport=dt_socket,address=8000,server=y,suspend=n

Then in Eclipse 3 click Run > Debug Configurations > create New Remote Java Application. Remember to attach sources. Invoke Maven bymvn jetty:runand start Debug in Eclipse.