After reviewing the E3 Gameplay Demo, I attempted to dissect what I could rather than a general level of viewing. Seeing the magic made me much more relaxed with Skyrim and, after scrutiny, I noticed a piece on dual wielding also.
Many feared that in Skyrim you wouldn't be able to block using dual blades or other weapons. (Leaving you terribly vunerable) But from what I see on this http://www.youtube.com/watch?v=ic6dKnv3WdU at exactly 7:32, after slashing the enemy's throat he blocks momentarily.
This may only be for a weapon-weapon combination, or more specifically a blade-blade combination, so speculation will continue.

That certainly looks like a block to me.

I've noticed this as well, good observation, because it is definitely an attempt to block or parry

I'm just curious as to how you'd block with the button system as it is currently... maybe HOLD left trigger/left mouse button instead of clicking?

I have assuming holding both buttons was block for a while, as for shields its hold the shield hand for block and shield hand plus other for bash.

A better view of it is in http://www.youtube.com/watch?v=N5uNlJAgIO4 at 0:46. Not only do you get the proper sound, you can see it in HD.

Also worth noting: The NPCs having a conversation in the main entrance chamber display some nice body language. You see the Redguard bandit standing with his hands on his hips, shifting his weight a bit as he speaks. It looks much better than the robotic stand-ramrod-stiff-with-arms-crossed-making-the-same-gesture-repeatedly stance of Oblivion.

After watching the video Gregasaurus linked to, I'm wondering if the player attempted a finishing move that got blocked.

yea, cuz when he did the finishing move the left blade was held the same way as the "block"

You can also see, while fighting the same bandit, that he does a finishing move without being into third person. I don't know if that's been mentioned but I know people have been complaining about that possibility.

That's not how finishing moves work in Skyrim. Players don't attempt them. They happen automatically when you land a killing blow on the last enemy in the area, and they're purely for show. If it was a blocked attack that would have been what we'd see: a blow that bounces off the bandit's shield.

hmm... I thought it was a power attack on the last enemy... anyways, i hope we can guard dual-wielding. I guess not 100% necessary, but i would like some form of defense...

As far as I've heard you can block while dual-wielding or even while wielding a single weapon, but it's nowhere near as effective as it is with a shield. Only confirmed no-block scenario is spell + weapon.

That's what I thought too but I've not paid much attention since I'm not a huge fan of finishing moves.

It didn't look like a block attempt to me. I suppose when you block, the weapon will be in a position where it will give the impresion of catching any ongoing attack, but in this case any attack hitting the blades before you will be a bizarre angled one. On the other hand, if it's not a block what is it? Unless you can cancel power attacks I don't know what other move type could this be beside a block attempt.