We stayed at the Poly last year for our birthdays. I received a signed picture of Snow White as well as a palm frond basket with all kinds of goodies. My DS received a bunch of balloons with a cake, noisemakers, certificate, and hats. I hear that this kind of thing is hit or miss. Sometimes the resorts do something, sometimes they don't.
You can order a cake at the parks from Main Street Bakery and another place at the Boardwalk -- I think. However, if you mention it while making your PSs and also during checking in at the restaurant, most places do something. At the Biergarten, my DS received a cake, got sung to in German and also the entire restaurant toasted him. At Chef Mickey's, he received a placemat and a cupcake. At Whispering Canyons, he received a large chocolate cake thing -- for breakfast! The CMs do their best to make birthdays special!