Quoting Brian Rose (lists@brianrose.net):

> I just upgraded my file server (NetBSD 1.6.2) from a Pentium 200 to an 
> Athlon 1400. 
welcome out of the 90s :)

> I am looking for a simple way to shut down the machine by pressing the 
> power button on the front of the box. I have been looking around and it 

You can also get something to listen to (say) a second serial port.
DTR goes high (button pressed), script kicks off (and halts).

Perl can do it, but since unix does it, a "shut me down" program
called from /etc/ttys might cover you.