Honestly, I think I'd have to contradict that idea. The reason being is because for one, you get the Pikachu-Colored Pichu in DPPt from an event held because of the 12th movie. When you trade over the Pikachu-Colored Pichu to HGSS, and have it on your roster, you'll lead it to Ilex Forest I believe where you encounter the Notch Eared Pichu. I doubt having a battle with it and not being able to catch it will be the way it happens.
As for the Celebi, some have thought that, but then again I don't know how it'll really have much to do with Celebi. The reason being is because Celebi before was dealt with the GS Ball and for them to change it to do something with the Notch Eared Pichu will be kind of strange in my opinion. Though, it's still a bit early to conclude any of this, we'll just have to let time go by to see what it is all really for.