I have tried to track this information down and have found articles which have now confirmed that from November 2014 no ship exceeding 90 000 tons will be allowed to transit the Venice Lagoon and the Guidecca Canal which goes past St Marks Square. This will eliminate most of the Celebrity Fleet as the solstice Class are all over 120 000 tons, plus it will be a shame for P&O who have for the first time this year put the Ventura into the Mediterranean where she most definitely will be sailing into and out of Venice, she herself is also around the 120 000 ton mark.
But all is not lost, Venice can still be a port of call, it will just be via a different route. I have been lucky enough to sail out of Venice , and it is indeed an awesome sight.
So as you can see there is some merit to go to Venice on one of the big ships this year.