i ordered a birthday cake for my fiance at le cellier steakhouse in the canada ws at epcot for him on a recent trip. i made the request when we made the ps (priority seating) and it was $16.00, it was nicely decorated & chocolate & we had leftovers. you can order a specialty cake/dessert in advance for a fee at most wdw signature restaurants. call 407-wdw-dine to make a ps.
chef mickeys at the contemporary does alittle celebration for birthday & regular guests during its character meals. we did this for a cousin on a family trip, cm has a make your own sundae/cupcake area & he loved it.
i know they also do an anniversary table decor & cake at the royal table in cinderellas castle at magic kingdom for anniversaries, my parents did this years back & also got champagne glasses for the occasion. and wdw also has birthday packages aboard the specialty cruises, look under wdwinfo.com