I think this is the key:

http://www.weather.gov/sat_loop.php?image=ir&hours=24

If the cooler air (which is north and west of the big T-storms in the midwest) gets to the gulf coast before Gustav (which is off the map @ the lower right-hand corner), then that hurricane will likely drift to the west. The gulf energy platforms will be in Gustav's powerful North-East quadrant, but the shear would limit Gustav's intensity.

Hanna could turn more directly westward toward Florida. If there is an infill of cooler air, then the storm tracks would diverge, with Gustav tracking toward Texas then turning north over the plains and Hanna tracking west then turning north, perhaps moving out to sea without making landfall.

Official NHC has this:

http://www.nhc.noaa.gov/gtwo_atl.shtml

http://www.nhc.noaa.gov/refresh/graphics_at2+shtml/144513.shtml?5day#con...

http://www.nhc.noaa.gov/refresh/graphics_at3+shtml/144212.shtml?5day#con...

If the cooler air doesn't fill in, then Gustav will steer more towards New Orleans/Mississippi but will create shear that will limit Hanna's intensity.