Oracle Advanced Queuing LIFO
Assuming that you are using PL/SQL to dequeue the message (there may be additional JMS limitations-- I don't know enough about the interplay between JMS and AQ), you should be able to use a priority queue to implement LIFO (though it would be a bit hokey).  When you enqueue a message, you can specify a priority


  The priority attribute specifies the
  priority of the message. It can be any
  number, including negative numbers. A
  smaller number indicates higher
  priority.


If you create a sequence that counts back from some really large number, you could assign an ever increasing priority.  Then your prioritized dequeue would get messages in a LIFO order.