Im not sure how I feel about mega evolution. On Serebii it says [QUOTEThey "evolve" through a process called "Mega Evolution"[/QUOTE]. That makes me think its permanent should you choose to do so. I would also suspect that the mega forms would need to provide a serious power boost. If they have to hold an item to mega evolve, and only during battle, that would ruin strategies like using rest to heal, then having a held berry to immediately wake up. The addition of an item that creates a mega form would hinder held items in a competitive scene.
Pretty much in competitions, it would be either A mega form or B held item. I have a feeling it would be one of the options is chosen vastly more then the other. That is why I think its a permanent, outside of battle thing. Too many good held items to just go "nope, gotta hold this stone of a better form". Personally, I think its cool. I would like to know if All pokemon get a mega form. I really wanna see my glaceons mega form. I also want to know if just any pokemon can go into a mega form, or if they need to be pokemon with unusually good stats. Sort of like what people get for EV / IV training. That would make the forms a bit more prestigious.