Pokemon Diamon & Pearl Version are said to be released in Japan November 2006.  It will probably be for Nintendo DS.  The only information we know so far is :

It will be set in a new region

It has all of the DS's features (wi-fi, microphone, touchscreen, ect.)  Whether or not it will use the international clock and calendar is unknown

It will be compatible with Pokemon Ruby, Sapphire, and Emerald

There will be no new types, but there will be new type mixes

There will be many new Pokemon, including evolutions and pre-evolutions of old Pokemon

Up to 16 players can connect with the DS's wireless capabilities

Here are a list of all of the new Pokemon :

MUNCHLAX (the pre-evolution of Snorlax)

LUCARIO (a legendary Pokemon)

BONSLY (the pre-evolution of Sudowoodo)

MANENE (the pre-evolution of Mr. Mime)

MANYULA (the evolution of Sneasel)

MANAPHI (the legendary "Prince of the Sea")

PERAPPU (a bird Pokemon)

BUIZERU (a sea weasel)

TAMANTA (the pre-evolution of Mantine)