The 4.0.3 roms are based off from the uncompiled 4.0.3 source that google releases to the public, which developers can compile and test their apps on.
As for your 4.0.1, is your device a yakju, or jakjuux? Check with the GN checker app on the market. If its jakjuux, it wont be updated until samsung says so, meanwhile if you are running jakju, you should be able to go to 4.0.2, which has been released as an OTA through google. Hope this helps.
Sent from my MZ604 using xda premium
Nexus 5 - 4.4,
HTC One Developer Edition - 4.3
Nexus 7 3G - 4.3