From Bulbapedia, the community-driven Pokémon encyclopedia.
Pokémon XD is the tentative title to the new Nintendo GameCube game slated to come out some time in 2005. The game, being developed by Genius Sonority, is reported to be a full RPG similar to that of the Game Boy Advance games. Not much else is known about the game at the current time aside from screenshots that depict Pokémon Colosseum 3D character models.
The title is considered to be rather strange, and it's reasoning is unknown. It is unlike Pokémon to have meaningless names. One theory is that the Roman numeral version of XD is 490, and this could very well be the total number of Pokémon in Generation IV. However, it's more likely that XD refers to some part of the story.
Is it as of yet unknown if Pokémon XD will include new Pokémon, screenshots so far have only shown Pokémon from previous generations. However, with the fact that the game is not likely to appear for a while, and the inclusion of Munchlax already into Pokémon Dash, the game will probably include a full set of Generation IV Pokémon.