Had the same/similar issue and resolved it.
So yesterday I upgraded to Ubuntu 12.04. Everything seemed to go on fine...however, when the system rebooted, both keyboard and mouse did not work at the login screen. Tried different things (rebooting system without connecting keyboard/mouse, connecting it later on etc...but nothing worked).
In the end, pressed the "Shift" key when the system reboots to bring up the menu (that has options to boot into recovery mode, prior linux versions etc)..and..
Booting into recovery mode did not help.
Booted into a prior linux version. Login screen was displayed, and mouse/keyboard worked. Thus, i could login to the system. I noticed that there was a notification that there were certain unmet dependencies. Opened up a terminal and typed in the following command:
sudo apt-get -f install
This installed a bunch of things. Rebooted the system once the above command executed successfully. This time I could login successfully as the keyboard/mouse was working.
On login, there was an error (do not remember what it was). I opened up a terminal window and executed the following command
sudo apt-get -f upgrade
Took a few minutes. Rebooted the system once the above command executed successfully.
System is operational again. No errors.
Hope this helps someone. Thanks!