Hundreds of people celebrate a birthday at WDW each day. You can get a free birthday pin at Guest Services. Some of the CMs will give a little more attention to the person wearing the button. Sometimes, sit down restaurants will offer a desert to a birthday person.
If you mention it to the front desk of the resort, sometimes they will put a couple of balloons or an autographed character picture in your room. \
Bottom line, don't count on anything happening. It is all very hit or miss depending on the CMs you interact with and the mood of the day. If you really want something special for the birthday person, purchase it. You can get some nice things through the Disney Florist or you can order a cake through Disney.