Currently, the only legendary pokemon that Japan has that we don't is Shaymin. (The Shaymin event has happened over in Japan ; it hasn't happened over here yet.)
Other than that, Japan doesn't have anything that we don't (other than an unlimited supply of Celebi thanks to the Japanese Pokemon Colosseum Bonus Disc, although I have that.) (Japan also has Pokemon Ranger 2 which allowed them to obtain Darkrai once again, but that releases here later this year.)
Arceus, pokemon #493, has not been released by Nintendo anywhere in the world yet.
However, Pokemon Platinum will be released in Japan in a couple weeks (the 13th, actually) and will include...
-A new Giratina form, the Origin Forme. (Ability changed to Levitate, most likely more attack based... Outrage confirmed as one of its moves.)
-A new Shaymin form, the Sky Forme. (Ability changed to Serene Grace, type changed to Grass + Flying, confirmed to have new moves such as Air Slash and Leaf Storm.)
In addition, Nintendo has (within the last month) began hinting at
more new pokemon in the game. These are believed to be different kinds of Rotom, although that is only a guess, and Rotom is not generally considered to be a legendary pokemon. Whatever they are, we supposedly won't know until the 28th.