All sorts of Pokemon events have been announced recently for the US. I got my info from bulbanews, serebii.net, and/or pokemon.com.

Gamestop Shiny Legendary Beasts Event (Need Diamond, Pearl, Platinum, HG and/or SS)
Raikou (Jan. 3 - Jan. 9)
Entei (Jan. 17 - Jan. 23)
Suicine (Jan. 31 - Feb. 6)

Toy's 'R Us Ash's Pikachu Event (Need Diamond, Pearl, Platinum, HG and/or SS)
(Jan. 30 - Feb. 5)

Gamestop Celebi Event (Need Diamond, Pearl, Platinum, HG and/or SS)
(Feb. 27 - Mar. 7)
*This event has been leaked to bulbanews/serebii but has no official confirmation from Pokemon/Gamestop, so this is technically a rumor.

Hopefully these events will have Wifi versions too for those not in America (or Canada?) and Gamestop/Toys R Us-less, but nothing has been announced yet, unfortunately.

European Pokefans rejoice! The legendary beasts event from Gamestop in the US is coming to Wifi. Shiny Raikou, Entei, and Suicine will be available to download from the Wifi connection starting Feb. 7th for DPPt/HGSS. When these Pokemon are transferred to Black/White you can unlock Zoroark.

It has also been announced (according to Serebii) that Celebi will be made available "soon" in Europe as well.

Thanks for the info Token Girl. My son has very recently started talking about these events (he heard about them from a kid at school). How do these work exactly and/or what is the procedure for getting them? We'll probably be going to one of the Game Stop events.

Full instructions are available at the main pokemon website (and probably the Gamestop employees should be able to help too). The short version:
1) Go to gamestop
2) On the main pokemon game menu (where you can select continue game or new game), choose mystery gift.
3) Select receive gift from wireless. The pokemon should then download onto the game.
4) Start your game file
5) Making sure you have a space in your party, go to any Pokemart. There will be a delivery guy in green who will give you the gift.
6) Save your game.

That's pretty much it. There are also events done over the Nintendo Wifi connection that you can download anywhere. For those events, just select the "receive gift from Wifi connection" instead of wireless from the mystery gift menu.

One last thing to remember: with each downloaded event pokemon or item you will receive a wonder card. This shows that you have done the event. Your game card can only hold 3 wondercards at a time, so when you get more than that, you have to delete your old ones.

Thanks for the tips. I poked (no pun intended) around the website and figured I'd post the link to the legendary 3 site. I found the instructions, that jive with what you've already said ofc, at the bottom right of the page.

http://www.pokemongoldsilver.com/us/#/news/trio/

The legendary beasts one is happening in Canada during those same dates. They have it up on the gamestop.ca site right now.

As for the other two I'm not sure if those'll be in Canada. Not really sure I care about the pikachu but I'm hoping the Celebi will be a Canadian event as well.
Someone on another forum (the Serebii one I think) had said that the Celebi event was on the same cart as the legendary beasts. I don't know if that's true or not since they didn't backup why they could say that but regardless I don't see why they wouldn't have the Celebi event over here as well.

Here's a confirmation for Celebi. Celebi will be distrubted in the Pokemon Black/White Mall tour. If you want I can post the times and malls they go to.

The Celebi event will be at GameStop starting March 6. It doesn't have an end date though. Hopefully it isn't one day only.
Source

EDIT: Celebi will be distributed at GameStop from February 21 to March 6
Source

Does anyone know if there's going to be any more Wi-Fi events for HGold/SSilver now that B&W has been released?

Er... We still have Pokewalker events and the Azure Flute, but I really doubt that the latter will ever come out. The former may, but they would have been out already, seeing as the last one was near September. And there's also the Lock Capsule, but again, not that likely.

In short, probably not.