USB mouse: mouse wheel not working

[Kai Grossjohann <kgrossjo@eu.uu.net>]

I'm running unstable.  I'm very fond of "dpkg-reconfigure
xserver-xfree86" as it is very easy to get a working X11
configuration with this.  Alas, this time it didn't work too well:
the mouse wheel for my USB mouse is not working.

Here is what's in /etc/X11/XF86Config-4 regarding mice:

Section "InputDevice"
	Identifier	"Configured Mouse"
	Driver		"mouse"
	Option		"CorePointer"
	Option		"Device"		"/dev/psaux"
	Option		"Protocol"		"PS/2"
	Option		"Emulate3Buttons"	"true"
	Option		"ZAxisMapping"		"4 5"
EndSection

Section "InputDevice"
	Identifier	"Generic Mouse"
	Driver		"mouse"
	Option		"SendCoreEvents"	"true"
	Option		"Device"		"/dev/input/mice"
	Option		"Protocol"		"ImPS/2"
	Option		"Emulate3Buttons"	"true"
	Option		"ZAxisMapping"		"4 5"
EndSection

Section "ServerLayout"
	Identifier	"Default Layout"
	Screen		"Default Screen"
	InputDevice	"Generic Keyboard"
	InputDevice	"Configured Mouse"
	InputDevice	"Generic Mouse"
EndSection

Running xev shows me that the X server never sees any mouse-4 or
mouse-5 events from turning the wheel.  (It doesn't see any events at
all, in fact.)

One week ago, on my old laptop, I had the very same USB mouse working
well.  However, I was running another kernel at that time, and I
can't be 100% sure that the XF86Config file was exactly the same.
I'd be willing to bet a Pizza, though, that it was the same ;-)

Note that the mouse works, and the wheel functions as a mouse button,
but turning the wheel does nothing.

What can I do to diagnose the problem?

Kai