With i386 3.0_BETA, I've found that having a USB mouse, but no PS/2 mouse
kills the (PS/2) keyboard in X (it works from the console, but you can't 
console switch once X has started).

XF86Config snippets:

Section "InputDevice"
 	Identifier	"Keyboard0"
 	Driver		"keyboard"
EndSection

Section "InputDevice"
 	Identifier	"Mouse0"
 	Driver		"mouse"
 	Option		"Protcol" "wsmouse"
 	Option		"Device" "/dev/wsmouse"
EndSection

From dmesg:
pckbc1 at acpi0 (PHP0303): kbd port
pckbd1: io 0x60,0x64 irq 1
pckbd0 at pckbc1 (kbd slot)
pckbc1: using irq 1 for kbd slot
wskbd0 at pckbd0: console keyboard
pms0 at pckbc1 (aux slot)
pckbc1: unable to establish interrupt for aux slot
wsmouse at pms0 mux 0
wsdisplay0 at vga0 kbdmux 1: console (80x25, vt100 emulation), using wskbd0
wsmux1: connecting to wsdisplay0
wsdisplay0: screen 1-3 added (80x25, vt100 emulation)
wsmouse1 at ums0 mux0

End of dmesg:
pckbport: command timeout
pms_enable: command error 35
pckbport: command timeout
pckbport: command timeout
pms_disable: command error
pckbport: command timeout
pckbport: command timeout
pckbport: command timeout
pms_enable: command error 35
pckbport: command timeout

This is very different to the dmesg when the PS/2 mouse is plugged in:
pckbc1 at acpi0 (PNP0F13): aux port
pckbc1: irq 12
pckbc2 at acpi0 (PNP0303): kbd port
pckbc2: io 0x60,0x64 irq 1
pckbd0 at pckbc2 (kbd slot)
pckbc2: using irq 1 for kbd slot
wskbd0 at pckbd0: console keyboard
pms0 at pckbc2 (aux slot)
pckbc2: using irq 12 for aux slot

It looks like ACPI is conspiring to cause problems.

(This may be related: the kernel panicks when no keyboard (type 
unimportant) is present and X starts).

-- 
Stephen