The following code used for unpacking bitmap data produces bad results on ATI 7.6 on Vista, while on XP (both NV & ATI) works quite well.
This should teach me about applying shortcuts by using less frequently used OpenGL driver codepaths. :S
float index[] = {0.0, 1.0};
glPixelMapfv(GL_PIXEL_MAP_I_TO_R, 2, index);
glPixelMapfv(GL_PIXEL_MAP_I_TO_G, 2, index);
glPixelMapfv(GL_PIXEL_MAP_I_TO_B, 2, index);
glPixelMapfv(GL_PIXEL_MAP_I_TO_A, 2, index);
format = GL_COLOR_INDEX;
type = GL_BITMAP;
glTexSubImage2D(target, 0, x, y, xsize, ysize, format, type, data);