Here is the answer I received from Disney.
Dear Jeff,
Thank you for your interest in a
Disney Cruise Line vacation!
I am happy to assist you.
Good News! Although Catherine's Palace is normally closed on Tuesdays,
Disney Cruise Line has made special arrangements to provide a port
adventure to include this location.
As a reminder, based on availability, the following Port Adventures
(featuring Catherine's Palace and Hermitage) will only be held on July
13, 2010:
-Catherine's Palace & Hermitage
-Hermitage & Peterhof
-City Highlights & Hermitage
-Catherine's Palace & Hermitage Museum
The following Port Adventures will be held on both July 12 and 13, 2010.
-Hermitage Museum
-Hermitage Museum & Gold Room
-Catherine's Palace in Pushkin
The following Port Adventure will only be held on July 12, 2010:
-Royal Ball at Catherine's Palace - Exclusive Disney Character
Experience
I hope this information has been helpful. If you require further
assistance in planning your visit, please feel free to contact us.
We look forward to sailing with you!
Have a Magical Day!