We also have celebrated an adult birthday at Disney. When I made the ps, I had his birthday noted on our two reservations for that day and also for one later in the week at Akershus b/c I knew my dh would love having those beautiful Norwegian women singing to him. At 1900 Park Faire breakfast, they gave him a decorated over-sized cupcake. When they brought it out, the server (or maybe Alice -- can't remember) had everyone in the restaurant to sing happy birthday to him. Then all the characters visited again. Lots of fun. At Boma, he also was given a huge decorated cupcake, but no fuss. At PF, I reminded the hostess that it was his birthday. At Boma, I told the hostess and the waiter.
Our lunch at Akershus was several days later, and I had forgotten I had asked for birthday treatment, so I was as surprised as him when all the servers in the restaurant came with a cake (not cupcake), a certificate and a wonderful Norwegian song. He loved it! And they even boxed the remainder of the cake for us to take back to the resort.
We also got special treatment at our resort, Yacht Club. I sent a very nice fax with a birthday cake on it before arrival. I didn't ask for anything specific, just that his birthday be as special as possible at our favorite resort. We were upgraded from standard to water view and balloons and a card were sent to the room.