Most people probably now know about the mysterious splash page that has been spurring much conversation. Originally, it appeared only on the main Blizzard website, but now it seems to be on all Blizzard websites, including the main WoW site.
Basically, it's a big picture of ice - that's it. Some people are noticing "hidden" runes and pictures in the ice, but I don't really think it means anything - it's just coincidence. The main discussion seems to be a Diablo announcement vs. a WotLK announcement for the release date or beta. Now, my personal opinion is that it is an announcement for WotLK, but it's not for something as small as the beta and it's too far away for an actual release date (for Blizzard anyway). I think we'll be seeing a HUGE announcement for WotLK: another new hero class, another new profession, something completely new, etc.
Now, there is always the possibility of it being for a new franchise. However, I don't see Blizzard doing that right now because it could potentially take hype away from Starcraft II and WotLK. Plus, it's a picture of ICE for goodness' sake!