That won't stop us from guessing what's in it it. Our guess: a lot of stuff. Last year's bag was huge, as you can see in the gallery below, and this year's will probably be even bigger. There will definitely be some silly items, like the QQ noob tissues and the Diablo III mints from last year. There'll be promotional stuff for sure, maybe a pack of TCG cards, a few licensing partner ads, and/or a sample mini (last year they gave out Voidwalkers). And of course there'll be an ingame item card, maybe a sweet mount or a vehicle. Personally, I'd love to see a special edition novel, either a reprint of something like War of the Ancients or even a special collection of short stories just for the event.
Whatever's in the bag, you can rest assured that we'll be one of the first few folks sifting through the goodies again. And it's not outside the realm of possibility that, yet again, you'll have a chance to win one. Stay tuned.
Gallery: BlizzCon 2008 Swag Bag gallery