TipTip,
Your certificate will read the highest of any rating... For example, i have taken my ATP checkride in a Multiengine aircraft, but not in a single engine aircraft, so my certificate reads:
Airline Transport Pilot - Airplane Multiengine Land
Commercial Privileges - Airplane Single Engine Land
The "instrument airplane" portion is removed since ATP standards require instrument proficiency at or above those for the instrument checkride. (IE it's assumed that you can fly IFR in the category aircraft you have an ATP rating for).