well it seems like we have ruled out almost everything...

Try this, just try using your cubemap as a projected texture, just to test to see if its really being turned into a cubemap.

To do that, just calculate some coords in your vertex shader like this:
projectcoord = VertPosWorld - lightposition;

then just use texCubeProj() in the pixel shader..

Thn just set your final color to this color.. only for testing to rule out any problems with the cubemap itself.. (like maybe its backwards or not being turned into a cubemap for some reason..)