So, predictions for these Events?
Arceus - This gives you a choice of the three dragons (Dialga, Palkia, Giratina) I imagine. They'll be created or 're-born' by Arceus, hence the level 1 setting.
Pichu - Who knows, possibly something to do with Celebi? I'm expecting this to be a useless event though really.
Jirachi - I'm going to guess that using Draco Meteor in a special area of the game will let a meteor strike the Pokémon world (somewhere, maybe Mt. Moon). This will be Deoxys, and you get to capture it.
That leaves Celebi needing an event (maybe), which I expect to be a Wi-Fi one. Then we have every legendary available on 4th gen games (including Ranch, Shaymin event which was worldwide, and the Ranger games) except Mewtwo, Latias, and Latios (and Arceus). Surely those three will be in HeartGold / SoulSilver? Mewtwo back in the cave, and one Lati roaming per game (opposing colour to Kyogre / Groudon I'd guess at, like in R/S). (Arceus, worldwide event hopefully).