Filb.de has uploaded a video clip from Oha Suta, a Japanese TV show, which shows footage of the Notched-Ear Pichu event in Ilex Forest and the Arceus event involving Cynthia. Thanks go to Yaminokame and Bangiras for the translations!
For the Pichu event, Notched-Ear Pichu will join your team in Ilex Forest once you have traded over the movie “Pikachu-Colored Pichu.” It is female and will know the attacks Helping Hand, Volt Tackle, Swagger, and Pain Split. The video shows a battle between Igglybuff, Marril, and both Pichu in a Union Room match to showcase what the Pokemon look like. The clip reveals updated Gold and Silver battle music and that the HP bars are white once again.
The Arceus event clip shows Gold walking with his Arceus into “Shinto Ruins,” a hidden location. The exact whereabouts of the ruins within HeartGold and SoulSilver are unknown, but because snow is falling it could be on or near Mt. Silver or even back at Mt. Coronet. When you trade over the movie Arceus to the games and bring it to the Ruins of Alph, the location will unlock itself. Cynthia will already be at the ruins waiting for you, perhaps because she is still studying the legends of Sinnoh. Your Arceus will tell you that you can pick one of three Pokemon for it to create, being Dialga, Palkia, and Giratina. Once you pick the Pokemon you want, the Unown will swirl around you, an egg will hatch, and the Pokemon you picked will apparently appear in your party at Level 1.