Celebrity does not usually enforce the dress code - but there have been instances where it has been. (I have personally seen someone turned away from the theater on a formal night who changed after dinner on a formal night.) To say "Celebrity does NOT enforce the dress code" is a disservice to others who may take that as the law and be on the one particular cruise and/or night where the dress code is enforced.
When people ask me about dress code, I usually reply that Celebrity has three types of evening dress which according to the daily is required in public areas of the ship for the evening with the exception of the pool deck. The enforcement of the dress code is sporadic.
That leaves it to them to make the decision about what to do.
Millennium-Transatlantic-9/01, 10/03, 11/05
Zenith-Special Caribbean 11/06
Back to back Millennium 28 days - Mediterranean 11/18/07, Transatlantic - 12/2/07