In Mint11 and earlier, the easiest way to switch users via the menu was to select Menu / Logout / Switch user.
In Mint12's MATE, selecting Menu/Logout results in a '(Switch user) (Cancel) (Logout)' menu popping up. However clicking on the first button appears to do nothing. In order to switch users, you have to do 'Menu / Lock screen' and then switch user from there.
What would be really nice is a working 'switch user' in the menu, of course...
(Mint Linux 12/Lisa 64-bit.)