It's a subtle change, but a definite one -- and it's one that not everyone seems to be able to see. However, at least for some folks, portions of the official forums have started to switch over to the new Wrath of the Lich King theme. Mostly, it's the menu and organizational pages showing the new theme for certain viewers.
This may be a bit of Internet turbulence -- it could be something that Blizzard hadn't really intended to release yet. There was quite a bit of spotty login troubles earlier this morning, and several people reported having difficulty reaching the forums. This hint of what's-to-come could be an error, or some kind of forum glitch. But, at least for now, we can get a sneak preview of what our forums are going to look like while we face the Wrath of the Lich King.
Edit: Within 20 minutes of the theme being visible, it looks like it may already have been reverted. We hope you got a chance to see it!