Mmm.. as far as I remember they don't do anything special for this day... Two years ago they did a small celebration in the UK pavilion (my ex-roomate worked there and they made him wear a very silly costume and he just gave little shamorock stamps to the kids).
Very near Downtownd Disney you can find a Bennigans, and I remember they trhow a huge party on the parking lot... with green beer!!:veryconfu