I'm sure you all know that by now, so I'm not going to go into great detail. However, I will admit that these were about the last Pokémon I expected to have a mega evolution, because they already have their species-specific item with the Soul Dew, which grants stat boosts that add up to around 150 base stat points, compared to mega evolution's flat 100. What they intend (or intended, rather, since their code is already written) to do with these remains a mystery, though. Three major possibilities spring to mind, though: a typing change, an ability change (that would radically change the way they're played), or something to differentiate the two more than they are now, like what the Therian forms did for the genie trio. Or any combination of these three, really, since they're not mutually exclusive. Or maybe it's an attempt at spicing up OU play with them, which may well backfire considering that they're huge powerhouses from the start, and their Mega Stones can possibly join the Soul Dew on the banned item list.
Unfortunately, it's unlikely that we'll find out any of this any time soon, because I've been told by a few people that... well, I'm utterly inept with technology, so I couldn't make out jack, but what I can gather from what little English these paragraphs contained is that we can literally only know the appearances of the event Pokémon and Mega Lati@s. There's no way to chuck Master Balls at them and stuff them with Rare Candies to see their base stats, abilities and so on. So as it turns out, actual datamining is still a long way away. But hey, that's a start, I guess.