kernel hangs at serial port during boot

[walt]

sio0: configured irq 4 not in bitmap of probed irqs 0
sio0: port may not be enabled
sio0 at port 0x3f8-0x3ff irq 4 flags 0x10 on isa0
sio0: type 16550A
<boot hangs forever at this point>

The previous kernel compiled on July 22 worked fine
and I've made no changes to the kernel configuration.

I'm using the standard GENERIC.hints files copied to
/boot/device.hints, and the previous kernel used irq 4
for the same device with no complaints.

I have different machine running today's current kernel
which is perfectly happy with the serial ports so there
must be something funky about this machine, but I don't
know what.

Hmm.  I see that my working machine prints this dmesg:
"sio0 port 0x3f8-0x3ff irq 4 on acpi0" so perhaps the
difference in machines has something to do with acpi?

Anyone else seeing this?