I have narrowed down the problem. The 1 pixel offset occurs in windowed or full-resolution modes only. On my 1440x900 monitor, in video mode 1024x768, the offset becomes extremely noticable like in my picture. Can you confrim this by changing the video res in the source and running? I don't think the problem is the cubemap function (as you said it is the same as the normal dx one) but rather with the viewport size variables or something related.
-mike-

http://www.groundtacticsgame.com/
Alienware m17x R Custom laptop
specs:
Intel Core 2 Extreme Quad CPU Q9300
2x Nvidia 280GTX 2GB vram
6GB ddr3 memory@ 1333Mhz
512GB SSD
1200p 17' screen
runs Crysis Warhead on max settings at 1200p at 90 fps