There are a few things to note, however:

1. Blizzard has known about Diablo3.com for a long time, but hasn't done anything until now.

2. Similar things happened with worldofwarcraft.com and starcraft2.com.

It might be somewhat big news, but don't take it as an announcement of Diablo 3. I would say, be on the look out for an announcement at WWI. If it doesn't happen at WWI, I'd be willing to say that it probably won't ever happen.