It looks like there is a secret level in the Billy Hatcher & the Giant Egg demo of the Nintendo preview disc that is available in Japan and USA and soon in Europe. Just hold down the A key while the game is loading and release it only when you've pushed the Start button to begin the actual play to access the hidden level. Beware that this level is not very different from the other one, so the differences won't be immediately noticeable.

The disc, in the USA, is being bundled with new Gamecubes or sold for $ 9.99, and contains the playable demos of Viewtiful Joe, Billy Hatcher & the Giant Egg, Sonic Adventure DX: Director's Cut, Tom Clancy's Splinter Cell, Soul Calibur II, the movies of F-Zero GX, Star Wars: Rouge Squadron III, Viewtiful Joe, The Legend of Zelda: Wind Waker, NCAA Football 2004, Madden 2004, Tom Clancy's Splinter Cell, Soul Calibur II, an ESRB rating guideline movie and two puzzles for a GBA - Gamecube connection, Wario Ware and Dr. Mario.