Ha, I think I see the problem now...
I'm using Ubuntu, which is based on Debian, and:
# /etc/modules: kernel modules to load at boot time.
#
# This file contains the names of kernel modules that should be loaded
# at boot time, one per line. Lines beginning with "#" are ignored.
lp
psmouse
...I use a ps/2 mouse, so I need the psmouse module to be started at boot.
If you used a USB mouse you'd need different modules in /etc/modules, 'usbcore' and probably some more. The modules file must be setup at installation.
I used to think hotplug would load modules for whatever mouse you use (I'm nearly sure HAL does - when it's started)...
Considering it obviously doesn't, both psmouse and the modules for a USB mouse should be compiled into the kernel for Debian IMO.
@cymon: you could add 'psmouse' to your /etc/modules file and remove the line from grub, if you'd prefer that..