My magic mouse (bluetooth) right click (alternate click) stopped working after upgrading to Mountain Lion. I tried the following: verified that the mouse settings were correct, rebooted, and rebooted with shift key to be in safe mode. I tried logging in as a different user on my iMac.
I tried using a USB mouse and the right click worked, so the problem seems to be associated with the bluetooth magic mouse.
The mouse is not at fault: I rebooted from a Snow Leopard hard drive and the right click worked fine. I also tried the magic mouse with another mac and the mouse worked fine.
My guess is that there is a library/preferences file with an incorrect or corrupted setting. If so, which one (e.g., com.apple.Bluetooth.plist, com.apple.Bluetooth.plist.lockfile, other ...)? And is it OK to delete the file?