If your mouse doesn't work on a Ubuntu Guest with Virtualbox, here's what seems to be the only way to get things working again.

fix_guest_additions.sh

#Virtualbox, Ubuntu and Guest Additions often get cranky, here's how to fix them so you're bloody mouse works again.
sudo apt-get install -f
sudo apt-get clean
sudo apt-get update
 
sudo apt-get install linux-headers-$(uname -r)
#OR
sudo apt-get install --reinstall linux-headers-$(uname -r)
 
sudo /etc/init.d/vboxadd setup