Comment 10 for bug 848562

I had the issue described above, and decided to give the nouveau driver a try instead of the proprietary nvidia driver I had been using. I cleared out all graphics card and monitor related sections from xorg.conf file and replaced simply with

Section "Device"
    Identifier "nvidia card"
    Driver "nouveau"
    Option "GLXBlank" "true"
EndSection

then launched in gnome-control-center and arranged the monitors next to one another. Now the lock screen dialog always shows up on the monitor the mouse was last set.