Well this is exciting. MMO Champion points us to WoW Wiki, where some enterprising person has assembled a map of Utgarde Keep, the first instance in Wrath of the Lich King. The map is apparently based on one published in Games for Windows, but as it appears on WoW Wiki, it's a little empty. Let's see if we can't clear some things up-- I've added letters to the map above, and below, I've described what is probably there, based on my own impressions and what we've heard elsewhere.
A. This is the main entrance, and as I said before, it consists of Vrykul metalworkers and forgemasters. The big firey graphic is the Forge, which has three "walls" that only open up after the mobs are defeated there. After you clear all three walls, you head around to the left, and find...
B. Dragonhandlers and "Trained Proto-drakes," which appeared to be Blue Dragons. Draw your own conclusions there. This area is a stable type of place, which explains the stalls.
C. This little arena is the first boss, Prince Keleseth, a Warlock that spawns skeletons to fight with him.
D. I didn't get past the first boss, but clearly it looks as though the dungeon's location changes here, either heading outside onto the mountain, or inside into some kind of natural caverns.
E. And at the end of the caverns is the second boss, Skarvald the Constructor. Obviously, according to his name, he may be overseeing some kind of construction on the "unfinished" part of the dungeon.
F. Finally, you continue around the dungeon to the last boss, Ingvar the Plunderer. There is a quest, given at the beginning of the dungeon, to defeat him, and given his name, he's likely a main figure within the Vrykul.
G. And afterwards, it looks like you'll be able to cut back, and (according to what I saw at BlizzCon) drop down back into the entrance area. Doesn't seem like a long instance at all-- took us maybe 10 minutes to clear to the first boss, so the whole thing probably runs about 30-40 minutes total.
All in all, should be pretty fun. Note that this is only half of Utgarde Keep-- we already know there is another instance in Utgarde, called the Catacombs, which hasn't even been seen yet. And this is only the first instance in the first area in Northrend-- there's lots more to come.