One idea i had is to put a lot of us in the abandoned tavern in Stormwind, and we can all look like we're interacting as people do in a tavern. And maybe slur out things about Cataclyschm. The reason i thought of it was that it fitted with our guildname so maybe they would think it was funny.
BUT, it's not an awesome scenery and perhaps if we think a bit we can come up with something funnier, or grander, or cooler, or i dunno.
Regardless of where we take the screenshot i think we should think about possibly wearing special outfits and items (like using feasts, or cakes or whatever else you can buy in Dalaran and plant on the ground/tables) and pets (maybe we all have kittens out if our scenery is kittenrelated for instance) and whatever. I dont think it works to just stand bundeled up someplace and saying "Cataclysm!", we need to be funny or clever somehow.
Oh now i had an idea that everyone can log on their druids and we can all be in catform with kittenpets and talk about CATaclysm.

(i thought of that when i read through my pet suggestion above!)
Well those are my 2 current ideas, they might not be awesome but maybe we can figure out something awesome along the way.