UPDATE 9/12/17 8.30am: Slightly later than we were expecting, Gen 3 spawns have now been switched on - meaning 50 new creatures are now available to collect in Pokémon Go.
The game's weather feature is also live, contrary to one earlier report which had it pegged for later in the month.
Several of the new Gen 3 creatures (Mawile, Absol) are now available in raids (with Shiny versions). Various critters are also available to hatch from eggs.
Early reports show Plusle and Minun are region-locked to south and north of the equator, respectively.
Leekduck.com has this handy graphic showing all of the Gen 3 available so far:
In addition to all of that, various existing Pokémon now have some of the new moves added to the game. Gyrados, for example, can learn the Fast move Waterfall, making it one of the best (if not the best) Water-type attacker.
UPDATE 7/12/17 9.40am: Check your phones - Pokémon Go's new update is live, although the game's Gen 3 spawns have yet to be switched on.
You can see if you have the new update from the new splash screen - it's all Christmas-themed.
While yesterday's announcement suggested the game's new weather mechanics were all set to go live immediately, a Niantic interview with The Verge suggests these will be activated later in the month, after the 50 new Gen 3 creatures arrive this week. (We expect them any minute now, in fact.)
There's a new look for gym battles, where you no longer need to hold down on the screen to activate your power move. Behind the scenes, meanwhile, code has been added for a new Star Piece item - a Lucky Egg-style consumable to boost your Stardust rate.
Fans have found a list of new Pokémon battle moves in the code as well, including Delibird's signature Present attack. The Christmas-themed penguin is one of just two Gen 2 Pokémon yet to see release.
There's also the ability to create Battle Parties - set groups of Pokémon to take into fights rather than having to organise your 'mons every time.
UPDATE 6/12/17 2.20pm: Pokémon Go's latest update adds 50 more creatures from Gen 3, along with an all-new weather-based gameplay mechanic.
There's no word on when the update will drop, although players now expect it imminently.
Among the 50 Pokémon included are Gen 3 starters Treecko, Torchic and Mudkip. A trailer below also shows Salamence, Shiftry, Aggron and Ludicolo. A legendary raid egg hatches to reveal Groudon.
More Gen 3 creatures will then arrive "over the next several weeks", developer Niantic stated.
Pokémon Go's dynamic weather will "change the way you and the millions of Trainers around the world discover, catch, and interact with Pokémon", Niantic continued.
Your in-game map changes to reflect local conditions, while Pokémon spawn based on their preferred weather. When it rains, for example, water, electric and bug Pokémon will be stronger, appear more frequently in the wild and offer bonus Stardust when caught. So, if it starts pouring, head outside for a better chance at a Mudkip.
Certain Pokémon moves will also be more effective dependent on the weather - such as Charizard's Fire Spin getting a buff when the sun is out.
We're asking Niantic now for more detail on when the update will go live.