En fait c'était ma souris qui était défectueuse...
sudo aptitude purge irqbalance
It seems that I've found a working solution (as suggested here: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-input-synaptics/+bug/549727).
Editing the relevant Gnome variable with the command gconftool -s /desktop/gnome/peripherals/touchpad/touchpad_enabled -t boolean true seemed to work to an extent.
What really finished the trick was using sudo modprobe -r psmouse; sudo modprobe psmouse proto=imps.
Some people have recommended just using sudo rmmod psmouse; sudo modprobe psmouse. That didn't work, while the above command did. I don't know what those extra arguments do, but they work.
Oh, and it still works when xserver-xorg-input-synaptics is installed, so you can reinstall that if you've taken it out. Also, new versions of the xserver-* packages have just been released, so upgrading is probably a good idea. I have, and everything still works.
One restart later and my mouse is working perfectly.
Thanks to the folks on Launchpad.
http://askubuntu.com/questions/130850/mouse-freezes-on-xorg-load
http://askubuntu.com/questions/85065/how-to-lock-touchpad-while-typing
org > gnome > settings daemon > peripherals > touchpad
Dans gconf-editor (sudo apt-get install gconf-editor) :
/desktop/gnome/peripherals/touchpad
à False
Toujours un problème (mais se répare tout seul) : (device number : 5, 6, 7, ....)
[ 2111.425022] usb 2-1.1: USB disconnect, device number 7 [ 2111.716652] usb 2-1.1: new low-speed USB device number 8 using ehci_hcd [ 2111.817195] input: Logitech USB-PS/2 Optical Mouse as /devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1.1/2-1.1:1.0/input/input20 [ 2111.817363] generic-usb 0003:046D:C03E.0005: input,hidraw0: USB HID v1.10 Mouse [Logitech USB-PS/2 Optical Mouse] on usb-0000:00:1d.0-1.1/input0
http://ubuntuforums.org/archive/index.php/t-1757679.html
xinput list