Let us hope they announce both. :)

If it indeed only Starcraft... then why the obvious Hel and Shael runes?

Well, only the Hel rune is truly obvious to be honest.

Also, why a "HUGE" announcement for a game already announced?

Only one rune is from Diablo, the other is Warcraft. And there may be hope yet, found this pick on Diablofans forum.

^ That middle pic is very close to what is being shown thus far, and I just stated, they said it was going to be a HUGE announcement and SC2 has already been announced so it wouldn't actually be HUGE, but more of just extra information.

Edit - Keep changing that pic and I have to keep changing my post to point at the correct pic, lol.

Sorry, wanted to resize it so that it all fit onto the screen.

^ That's fine, I was just being a pain in the ass. No need to apologize. :)

The fifth rune is a Viking Compass, and Blizzard did release a game about Vikings a long time ago.

edit:

from diablofans

The current splash screen looks like a protoss, or Arthas, but you never know.

Also:
Legit? What do you think? It looks to me like the brow doesn't match, unfortunately.

As much as I wish that was legit, sadly it isn't.

It looks like a zealots eyes in Arthas' helmet on Diablo's body, Blizzard is going to announce Warstarblo! The combination of all their game worlds into one ultimate game!

Anyways in all seriousness. It looks like Prottoss eyes to me but I'm hoping for the announcement of a new game rather than just something from a current one. Also the ice kind of makes me think it might have something to do with Wrath of the Lich King, since Northrend is the icy northern continent of the world of Azeroth and Blizzard has reused lots of things from Diablo in the WarCraft universe which would explain the rune from Diablo being there.