Okay, what is up with all the Legendaries? I mean, sure it's cool and all, but it doesn't make any sense. I mean in Platinum the only non-Sinnoh Legendaries are the Bird and Golem Trio, but there was a reason for them:
Bird Trio: Being Generation IV takes place at the same time as Generation II, it is well known that the Legendary Birds aren't in Kanto anymore because Red disturbed their nest and thye fled. Well, they had to flee somewhere and that somewhere turned out to be Sinnoh.
Golem Trio: I can answer why they are in Platinum in one word: Regigigas. Sure you needed the Movie Regigigas to activate the rooms to catch them in, but gamewise thinking I wouldn't be surprised that Regigigas (or the people who sealed it and the Golem Trio away) hid a set of Golem Trio in Sinnoh just incase they ever wanted to awaken Regigigas for an unknown reason.
However, I can't see storyline wise why Groudon, Kyogre, and Rayquaza would be in Johto. As for the 12th Movie Arceus activating an event which might allow you to catch a Level 1 Dialga, Palkia, or Giratina further confuses me.
Oh, and now with Jirachi I think that the Pokemon Company is going overboard, I mean give us Celebi who is the Generation II Cute Legendary, not Jirachi who is the Generation III Cute Legendary!