Still present in Mint 15. I'm using the Cinnamon edition (I know this is a MATE thread), so perhaps this is a Mint problem not specific to desktop environment or hardware. I'm running an AMD graphics card with the proprietary drivers, perhaps this is causing problems? Anyway this issue is compounded by the fact that not only MDM opens on the second screen (in my case the monitor on the left) but most program windows if maximised will open on the second screen as well. It seems to be that by default new windows will open on the second screen; loading screens (e.g. the LibreOffice loading screen) also start on the second screen, even if the window then jumps to my main monitor (as it is not maximised and I have moved it). This is not conducive to a usable/productive operating system!