I have seen lots of LAST_ACK states for TCP connections on my
   NetBSD 1.0A box.  When does this state clear itself out.  I have seen
   it stay stuck for hours.

Are these connections with machines on the other side of a PPP or SLIP
link, that may no longer be reachable?  It's conceivable that someone's
shutdown script is initiating a close sequence on a bunch of sockets and
then shutting down the link before the close sequences are complete.

It would be nice if the TCP code timed out LAST_ACK state after a while,
but I don't the TCP spec allows for this.

(Normally it would be cleared by the machine coming back up and sending
a RST the next time we retransmit our FIN-ACK.)