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So I went and looked up the dates on Gen 4 and 5 event Pokemon to compare them with the actual game's release.
Diamond / Pearl: Released Sep 2006 in Japan, and April 2007 in US
Manaphy - December 22, 2006 (Japan) and September 29, 2007 (US)
Darkrai - July 14, 2007 (Japan) and May 31, 2008 (US)
Shaymin - July 6, 2008 (Japan) and February 8, 2009 (US)
Arceus - July 11, 2009 (Japan) and November 7, 2009 (US)
For comparison, their respective movies were:
Pokemon Ranger and the Temple of the Sea (December 2006 Japan, and April 2007 US)
The Rise of Darkrai (July 2007 Japan and February 2008 US)
Giratina and the Sky Warrior (July 2008 Japan and February 2009 US)
Arceus and the Jewel of Life (July 2009 Japan and November 2009 US)
Too tired to do this for Gen 5, but if Gen 4's numbers are still relatively accurate, you should expect the first of the three in roughly 6 months, with the next 2 over the next 2 years. Of course, the dates are ALWAYS tied into the movies, and there's always translation time and such for those, so the very first one could be a bit longer.
