2 of 2 added 29 characters in body

I was able to reproduce the effect (in version 7 under Windows) but I don't yet fully understand it. Partly it appears when you have automatically-resizing Graphics objects contained in a Grid object that is wider than the Notebook window. For example, using this code:

blocks = Table[{Graphics[{Darker@Green, Rectangle[{0, 0}, {1000, 200}]}, 
     ImagePadding -> 0, PlotRangePadding -> 0]}, {i, 6}];

myGrid = Grid[blocks, Spacings -> {0, 0}, Frame -> All];

Style[myGrid, Magnification -> 3]

And in a Notebook window that is easily able to contain the Graphic I get a solid green area with only fine black dividers:

enter image description here

However, if I resize the Notebook window to a narrower-than-graphic size I see this:

enter image description here

However magnification also plays a role here. Using a fixed ImageSize, which prevents resizing, combined with magnification shows a gap even when the Notebook is plenty wide to contain it:

 myGrid /. g_Graphics :> Show[g, ImageSize -> 50],
 Magnification -> 4

enter image description here