Here are the relevant sections from my XF86Config-4:

Code:
Section "ServerLayout"
	Identifier     "XFree86 Configured"
	Screen      0  "Screen0" 0 0
	InputDevice    "Keyboard0" "CoreKeyboard"
	InputDevice    "PS/2 Mouse" "CorePointer"

/snip/

Section "InputDevice"
	Identifier  "PS/2 Mouse"
	Driver      "mouse"
	Option      "Protocol" "IMPS/2"
	Option      "Device" "/dev/psaux"
	Option      "Emulate3Buttons" "true"
	Option      "Emulate3Timeout" "70"
	Option	    "SendCoreEvents"  "true"
	Option      "ZAxisMapping"    "4 5"
	Option      "Device"  "/dev/mouse"
EndSection