The realms, they're back to workin'. After you've download patch 3.0.3, which is only 20 megs, you can log into the game and play around with some of the new features. We are not expecting very many realm problems tonight, since this was mainly a bug patch which fixed several outstanding issues.
However if there are connection problems tonight, we'll keep you updated with all the latest news from Blizzard.
So as long as you're able to get in and play, you'll quickly notice that the best part of the realms being up is that everyone and their cousin Niko is sporting the new Hallow's End title, "the Hallowed." If you weren't able to finish off everything necessary to get the achievement and accompanying title you'll need to wait until next year.
Of course you could always start worshiping the Ori. I hear they're pretty hallowed as well.