There wasn't really anything special - I believe that there was a special 'schools performance' was tacked on the end, and there was a cake etc, but the rumours are that it's the 21st year that they'll be celebrating in a big way (anyone heard anything else about that by the way?)....
The official website had a little piece of writing on it about the Hyde Park celebration last time I checked, and a box to stick your email address in for updates.
There will be a big anniversary celebration in Hyde Park this September. The idea is to have it on the Saturday after the Last Night of the Proms and use the stage/arena etc. all set up from that.