This is definitely some sort of Xorg bug. The xorg.log shows two different things in your working/not-working. One time is detects the mouse as serio and the other it detects as usb.
** Changed in: linux-source-2.6.15 (Ubuntu) Sourcepackagename: linux-source-2.6.15 => xorg -- Mouse does not work after kernel upgrade to 2.6.15-25 https://launchpad.net/bugs/49996 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
