The default display settings for gdm are fucked up. If they have a default for primary monitor, why showing greeter where the mouse is, which is in the middle, which depending on the widths of your monitor might end up on the wrong monitor (like in my case)?
Hell, GDM is even so ****ing stupid as to place the greeter window and panel on whichever monitor the cursor is currently on, including a "race condition" that allows the greeter to show up on one display and the panel to show up on another.
Sorry, but that's just retarded. I mean you. Not only can you place the screens using xrandr, you can do that in Display Preferences.
Oh, and just to top it off, the Linux r600 driver lists my displays in reverse order of Windows, so whenever I change OSes I need to swap cables to get the monitor that's actually in front of my face to be the primary display. Lovely.