A couple of years ago I wrote a paper on how to adjust the TCP settings so that TNS would detect a dead connection. I also demonstrated how Oracle cleans up processes, but may leave sessions connected to the database. ALTER SYSTEM DISCONNECT SESSION is the new command for cleaning up the session and process at the same time in 11g.
In the white paper, “Improve Performance with Dead Connection Detection”, I showed with 10gR2 how blocking locks are not released when the user session was abnormally terminated. The same test in 18.104.22.168 shows the blocking locks released almost immediately after the first session was abnormally terminated.
I believe this is due to the new process, dia0, dia(zero), in 11g.