This is probably just me being dim...
I have a laptop which I use as a firewall, it connects to a cable
modem and, by it's one serial port, to a modem.
For various reasons, I wanted a serial console for an OS upgrade. So,
as a stest, I plugged in my psion to the serial port and enabled it in
/etc/ttys, then kill -HUP 1. No problem.
Then I turned it off in /etc/ttys, HUPed init and plugged in the modem.
Unfortunatly, any attempt to connect to the modem (with cu or ppp)
then resulted in device busy errors (not certain of the exact wording,
I should have written it down).
I couldn't spot any processes which might be using the port.
In the end I rebooted ad so all is well,but I wonderd if anyone knew
what this was likely to have been.