I've been having issues with that.  When I go to remove containers on devicemapper (using the latest 0.6.5-dm-plugin code), whether they are running or not, I get that removeDevice failed, and it turns out that it fails precisely because the filesystems are mounted.  So do we need to umount more often, or do we just need to umount in removeDevice as a crutch until we get proper refcounting?