It took me awhile to get this to work so hopefully this will save someone some time. If you want to debug a webapp that is run using the mvn jetty:run or mvn jetty:run-war commands then these steps will help you get going quickly.
1) setup your webapp to run using jetty: Maven Jetty Plugin Guide
2) setup the debugger in one of the following ways:
(A) setup your MAVEN_OPTS (environment variable) to include debugging by adding this "-agentlib:jdwp=transport=dt_socket,address=8000,server=y,suspend=n".
Mine looks like this: MAVEN_OPTS='-Xms256m -Xmx512m -XX:PermSize=64m -XX:MaxPermSize=128m -agentlib:jdwp=transport=dt_socket,address=8000,server=y,suspend=n'
If you want it to suspend until the debugger is attached then just set suspend=y
Note that this will stop you from running more than one mvn process
(B) execute mvn using the mvndebug script (skip step 3)
The only issue with this is that it suspends always and you cannot kill it without attaching to it with a debugger (this annoys me so I use the first option)
Note that you can only have one mvndebug process running at once
3) Execute mvn jetty:run
4) Attach a debugger (like the one in eclipse) to port 8000 and you are ready to debug