serial hangs kernel

i've been having some boxes hang solid (only power cycle will work) when
trying to open ttyd0/cuaa0/cuad0.

the message
	sio0: port may not be enabled
gave me the clue, notice that the above is ambivalent. I enabled it in the 
bios,
and now it's working ok, at least on an IBM-R51, i still have to
check it on an IBM-T21 (that DOES have a serial port/outlet), and a
Intel/IBM blade.

so it seems, at first site, that if the sio hardware is on the MB, but
sort of not enabled - because there is no connector -, (why the parallel
port was kept and not the serial is beyond me), trying to open /dev/cuaa0 hangs
the kernel.

Q: in such case, shouldn't it not appear in /dev ?

btw, this is with -stable & -current

danny