I read a lot about troubles with ps/2 mice on a kvm switch (there's something screwed up with the psmouse.c in the 2.6.x kernels). Well, I did have those problems too and I read the fucking manuals and searched the fucking web .
With Debian Sid (I used sid before warty) I solved this in the german debianforum.de by editing my grub.conf:
root=/dev/hda1 "psmouse.proto=imps" roUnfortunately this works not with Ubuntu.
So I had to found another workaround for this: Just edit your /etc/modules:
Make sure there is NO "." between "psmouse" and "proto" and you have "mousedev" in your /etc/modules". Using exps instead of imps has advantages when you like to use your thumb buttons.