Has any famous (well... can't be famous without a reputation system and Skyrim doesn't really have that...) and powerful mage actually been challenged to a duel by another mage, yet? That was a small, early (believe it was said in the January edition of GameInformer) detail said to be in the game.
I was challenged by a mage in Whiterun, just outside of Warmaidens. I was 30th level or so (mage). He challenged me to a duel and we fought right there in the middle of town. I was Archmage at the time.
Also, I ran into a Volkihar Vampire out in the wild. Looked like a human in black robes, and it had a Vigilant of Stendarr as a thrall. They were standing over a body when I approached, and the fight was over very quickly. It was with the same character, probably 32nd level. It used lightning of some kind, and finished me off with an ice spike spell, but the spikes were huge, more like spears. It didn't look anything like that one picture we were all speculating was a Volkihar though, just a human wearing hooded black mage robes. I didn't get a really detailed look at it though, as the fight was over almost before it began, and it was night. It was definitely called a Volkihar Vampire though.
Add: @Sokar Rostau: Almost exactly where I fought my challenger! He was yelling the same thing at me, I summoned an Atronach and I had Jenassa with me. Your right too, he was tough.