I have a Logitech Dinovo bluetooth keyboard/trackpad that was working perfectly in Natty (11.04). After upgrading to Oneiric (11.10) it no longer worked at all and kept popping up a bluetooth connect dialog that consistently failed.
How to fix for 11.10:
How to fix for 12.04 (thank you QuadeHale)
gksudo gedit /lib/udev/rules.d/62-bluez-hid2hci.rules
Look for the line under "# Logitech devices" that starts with:
gksudo gedit /lib/udev/rules.d/97-bluetooth-hid2hci.rules
and change it to:
Leave the rest of the line intact.
Save the file and restart the machine. (You probably really only need to log off and back on).
If you are doing an upgrade, ( I just did 11.10 to 12.04), at the very end, when it's ready to restart, just do the edit as above before restarting. That way your keyboard/trackpad/mouse will continue to work after the restart. If you restart first, you will lose the bluetooth keyboard/mouse and you will have to plug in a wired keyboard/mouse to make the changes.
I have entered a bug on this at: