Thanks for the responses folks!  Andy, I ran into the same problem a 
while back (re. local subnet) and now have the same configuration (more 
or less) as you.

I just upgraded to 1.22 on both ends.  It took about 10 minutes for them 
to sort things out, but the tunnels are back up.  We'll see if i get any 
better luck this time.

If i implement something to detect the dead tunnel, is there a way for 
me to clear/reinstate it by poking something at m0n0?  If i bring the P2 
lifetime down (say 8 hours), what would the effect be?  Would it 
continue to reestablish the tunnel?