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 was considered to be rather strange, and it's reasoning was 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.
The game's full title appears to be: Pokémon Xtra Dimension: Dark Winds Dark Lugia.
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.
Currently there are three main characters: the Main Character(this is his current name), the female Sidekick, and Dark Lugia, which seems to be very important in this game, as it is mentioned in the game's title.