I have a 3-head X server without Xinerama.  (Xinerama doesn't support
mixed video drivers - in my case, nouveau + intel + nouveau.)  Thus,
DISPLAY is :0.0, :0.1, :0.2.

gimp Create -> Screenshot only captures data from its own screen.
Specifically, with gimp running on DISPLAY=:0.2,

- "Single window": gets the whole :0.2 screen if I select a window on
  any other screen.  (It does DTRT if I select a window on the :0.2
  screen.)

- "Entire screen": gets the whole :0.2 screen but not the others.
  (That is probably to be expected.  You can't count on the total
  workspace to be rectangular, or to use just one visual.)

- "Select a region to grab": As I drag a rectangle on :0.1, the
  rectangle appears in the same relative coordinates on :0.2, and that
  is what I get.

I doubt it matters, but in case it does: window manager is still ion3
from I think lenny/non-free.