My DH had his 39th birthday at WDW in Aug 2004. Our DD's 7th birthday was the same day - so I don't know if this affected the treatment he got. We started out the day with breakfast at CRT. Both DH and DD got birthday buttons at guest services in MK. They both received birthday cupcakes at breakfast along with certicates for their birthdays. Throughout the day, they both received birthday wishes from CM's and park visitors - though people usually noticed DH's button first - he's 6"4" and DD was 48" tall. She, however, made sure that everyone knew that it was her bday too! At Splash Mountain, the CM put them in the front of the log.
We went to LTT for dinner and again received your typical birthday attention.
When we returned to our room at POP, there were 2 sets of 3 balloons tied to special birthday cards from Mickey and Minnie, one set for each. I had mentioned at check-in that we were celebrating their birthdays on Aug 31st.
They had lots of fun! We will be there for their birthdays in Aug 2006 as well.