Now, I'm not so sure that is 100% truth or not, but I've found it pretty interesting.
People have read the data of the Smash Bros. Brawl disc and found unused and unfinished character data left on the disc. This data is stored with the other playable characters meaning that at one point, they were being worked on.
The following is the name of the character data:
They should be pretty explanatory. Dixie Kong, Dr. Mario, Mewtwo, Roy, Toon(?!) Shiek/Zelda. Pra_mai is Purasuru and Mainan, the two Pokemon from the newer games. This should effectively debunk the last minute Geno removal rumor. The last minute delay was probably because of sheer demand for the game, Nintendo had sold 1 million copies in only a few days in Japan already.
I'll be looking into it.
Here's a pic of the character files in the disk.