We had absolutely NO problem bringing an anniversary cake into Portobello Yacht Club in August.
Its funny; I contacted the restaurant first to ask if I could order one through Disney, and they said no...all they had were small 6" square 'celebration' cakes at the restaurant and they were not hooked up with the Disney bakery. They assured me, though, that I could bring one in as long as it required no refrigeration as they would not provide storage space.
At first, I wanted to order one from Disney and have it delivered there, or I'd pick it up and carry it in. I was told to contact the bakery on site to make the arrangements. I spent an hour at the GF trying to get this arranged and long story short, Disney wouldn't sell me a cake. I laughed, because here I was...willing to spend anywhere from $50-100 to get a nice Disney cake for my party of 15...and they wouldn't sell it to me because we were going to take it to Portobello.
So we went to Publix. They had a perfect (and DELICIOUS) cake, and even decorated it on the spot for me with 50th Anniversary decorations. $15.
The manager at Portobello was awesome. They took the cake from us when we arrived, and served it beautifully at the end of the meal...the staff came and sang Happy Anniversary to my parents, and then provided us with plates and a cake cutter for serving.