My mother tried doing this for my birthday last week and while she supposedly ordered a cake, got a quote for a price, plus tax, and handling charge, and gave her credit card number, I never got that cake. She had given our PS time and my name, etc... for it to be delivered after my birthday dinner. I was told by a Food Service Manager at the Poly(I think that was her title) that they dont take orders like that and if my mothers credit card is charged to call her and shell take care of it.
I did however get 2 complimentary cakes, LOL! One was exactly like Terk described(balloons, candles, and all) waiting in our room when we returned on our first night at the Poly. The other was after dinner at the Kona Cafe on my birthday. When DH had to ask about us not getting the "ordered" cake, they brought out a really nice cake with tropical flowers on it.
We are still waiting to see what shows up on my mothers credit card statement.