Previously I have had problem with mga driver 1.4.11 and xinerama extension on Arch linux, that I've resolved by manually patching the driver, using Ubuntu folks patch. (if you need xserver-xorg-video-mga-1.4.11, check out my previous post)
After an upgrade to kernel 3.3 the driver stopped working, as new version 1.5.0 has come out, obviously without the patch. It took some time, but finally I've got it working.
You can find the patched driver here.