According to the official Japanese Pokémon web site, trade via the Global Trade Station between US and Japanese versions of Pokémon Diamond and Pearl will be possible. Players will be able to determine the location of the original trainer of each Pokémon, and traded Pokémon will retain their nickname in full. Additionally, globe in the GTS will be upgraded so that the world map is accessible. The site also claims that foreign Pokémon will gain experience at an even faster rate than normal traded Pokémon.
Read more on Bulbanews