Guest relations will give you a button to wear. We were there for our 4th anniversary this year, and got many happy congratulations, some choice seats on rides, and since we were traveling with another celebrating couple, the CMs at Splash Mountain gave us a boat of our own! We also got a choice table for an anniversary lunch at San Angel Inn in Epcot right by the river. I don't know if it because of the buttons, but it certainly couldn't have hurt!
The couple we traveled with were celebrating a belated honeymoon-his vehicle broke down and they had to spend their honeymoon money to get it fixed, so this trip was about a year late. They also wore buttons, and were given a beautiful certificate at the American Experience in Epcot commemorating their honeymoon. It's framed on their living room wall!