It's possible I'm just doing something very stupid, but can't figure out
what it is.  Probably something with the kernel configuration as I'm not
exactly sure what all the ACPI-options are. With a self-built
ACPI-enabled kernel everything else works fine, but if a program tries
to access mouse somehow, the mouse and keyboard freezes completely. The
rest of the system keeps running nicely. With generic kernel, everything
goes well. In other words, starting X or wsmoused is impossible. The
mouse is a typical PS/2 one and rest of the hardware is pretty generic
i386 desktop stuff.

The things I can capture from dmesg are:

pckbport: command timeout
pms_enable: command error 35
pckbport: command timeout
pckbport: command timeout
pms_disable: command error

which happens straight away after starting wsmoused.

Differences to the GENERIC kernel configuration are:

#options        I386_CPU
#options        I486_CPU
#options        I586_CPU
#options        MATH_EMULATE    # floating point emulation
acpi0           at mainbus0
options         ACPI_PCI_FIXUP          # PCI interrupt routing via ACPI
options         ACPI_ACTIVATE_DEV       # If set, activate inactive devices
options ACPI_DISABLE_ON_POWEROFF        # disable acpi on power off
acpiacad*       at acpi0                # ACPI AC Adapter
acpibat*        at acpi0                # ACPI Battery
acpibut*        at acpi0                # ACPI Button
acpiec*         at acpi0                # ACPI Embedded Controller
acpilid*        at acpi0                # ACPI Lid Switch
acpitz*         at acpi0                # ACPI Thermal Zone
com*            at acpi0                # Serial communications interface
fdc*            at acpi0                # Floppy disk controller
lpt*            at acpi0                # Parallel port
npx*            at acpi0                # Math coprocessor
pckbc*          at acpi0                # PC keyboard controller

And if it is of any use here is acpi related kernel bootup messages:
# dmesg|grep -i acpi

acpi0 at mainbus0
acpi0: using Intel ACPI CA subsystem version 20040211
acpi0: X/RSDT: OemId <ASUS  ,A7V8X-X ,42302e31>, AslId <MSFT,31313031>
acpi0: SCI interrupting at int 9
acpi0: fixed-feature power button present
ACPI Object Type 'Processor' (0x0c) at acpi0 not configured
acpi: activated PNP0C0F
acpi: activated PNP0C0F
acpi: activated PNP0C0F
acpibut0 at acpi0 (PNP0C0C): ACPI Power Button
PNP0C01 at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0C0F at acpi0 not configured
PNP0A03 at acpi0 not configured
PNP0C02 at acpi0 not configured
PNP0C02 at acpi0 not configured
PNP0000 at acpi0 not configured
PNP0200 at acpi0 not configured
PNP0100 at acpi0 not configured
PNP0B00 at acpi0 not configured
PNP0800 at acpi0 not configured
npx1 at acpi0 (PNP0C04)
fdc1 at acpi0 (PNP0700)
lpt3 at acpi0 (PNP0401)
com3 at acpi0 (PNP0501-1)
pckbc1 at acpi0 (PNP0303): kbd port
pckbc2 at acpi0 (PNP0F03): aux port
PNPB02F at acpi0 not configured
PNP0C02 at acpi0 not configured

Cheers,
Juho