The newest snapshots introduced native fullscreen on Mac OS X. And I can't thank you enough for that. It eliminates my other issue:
But this introduced a minor bug to me. When going in fullscreen with the "two-arrows-button" in the upper right corner, the top of the window is grey. It looks like it is some sort of a bug with the menu-bar in OS X. I don't know if it's a LWJGL, OS X or Minecraft bug though. See attached picture.
Again, thanks for the native OS X fullscreen. So much better, especially with dual screens.