It's something that's been going on over the last two weeks, and I completely missed it, being busy with other things, chiefly Bravely Default, but here you go anyway in case you weren't aware.
Right, so it's not a massive change in methodology like the one from 2008, but it's still worth mentioning. Originally, they were using raw, unweighted stats to make tier lists. Then, in late 2012, they started assigning weights to each player's teams depending on their rating, which had the benefit of weeding out bad players, bad teams, and so on.
However, Pokémon Showdown's popularity kept exploding, which you can see right away in the monthly stats with the ludicrous amounts of battles done over it. This brought a whole new problem: the ratio of "casual" players to "competitive" players kept increasing, to such an extent that even stat weighting started having its limits. In his announcement post, which you should read if you want more detailed background, Antar cites the fact that no less than 4% of Sylveons ran Hyper Beam.
No, let me try this again: 4% of Sylveons ran Hyper Beam AFTER WEIGHTING. Anime-based teams are also even more popular than ever - even with Blastoise actually being usable this time around, its most popular teammate is Snorlax, and its fourth most popular PIKACHU. Tiers are supposed to be threatlists, and with such an influx of casual players that actually get to have an impact on weighted stats with Pokémon that just aren't threats, that definition is challenged.
This means that moving forward, the rating at which point your teams will really have an impact on the stats will be much higher. This month it'll be 1760, and they'll be winging it for a while until they get a more permanent solution in effect, which they're already working on. It sounds elitist, everybody acknowledges it, but when tiers stop serving their assigned purpose, something has to be done.
This change single-handedly moves around many Pokémon between tiers (rankings provided are the normal, weighted ones we're familiar with, not the 1760 ones):
Promoted from UU to OU: Chansey (#52), Latias (#59), Keldeo (#74), Zapdos (#82)
Promoted from BL to OU: Terrakion (#61), Kyurem-B (#63), Landorus-I (#65), Deoxys-E (#70), Deoxys-LG (#87), Manaphy (#88)
Demoted from OU to UU: Galvantula (#37), Cloyster (#39), Trevenant (#41), Sableye (#42), Klefki (#43), Donphan (#44), Tentacruel (#46), Starmie (#47), Smeargle (#48), Salamence (#49), Forretress (#50),