Shoko Nakagawa posted three pictures on her blog recently that shows a banner on the Pokémon Smash set that says: Pokémon Anime 15th Anniversary Special (ポケモンアニメ１５周年スペシャル). It appears that this is from a recent block of Pokémon Smash filming and the 15th Anniversary Special episode will most likely air on Sunday April 1st, 2012's episode of Pokémon Smash as it is the exact anniversary date of the anime which began airing in Japan on April 1st, 1997. It appears Rica Matsumoto (松本梨香) will be a guest as she is in a picture back stage with the Pokémon Smash cast. Hikozaru also appears like it will make a brief appearance as Shoko-tan took a few 'reuniting' pictures from the set.


Source: http://ameblo.jp/nakagawa-shoko/entry-11184556192.html