We found that when attached to a process with jconsole the value for "Used heap" was growing even though the main class of the attachee was a do-nothing-class (attached). The leak appeared to continue growing after jconsole was detached.

The leak did not reproduce with a recent HotSpot running on JDK6u29, so we looked at recent changes in src/classes/sun/management and tried un-applying e0c3fd538f1f
[ http://hg.openjdk.java.net/jdk7u/jdk7u-dev/jdk/rev/e0c3fd538f1f ]

The leak did not reproduce when running with the change reverted so this particular change seems to be a likely culprit for the leak but we're not familiar enogh with the management classes to be able to see exactly where the leak occurs.

We looked at a hprof dump (attached) of the leaking process and found j.l.m.MemoryUsage at the top of the heap histogram with 98430 instances.