This one, however, has actually fallen into the hands of the Scourge -- it was wielded by a female blood elf warrior named Lana'thel who fell in battle against Arthas and Frostmourne, and was raised back as one of the San'layn, a small faction of vampiric blood elves forced to serve the Lich King (we'll be fighting them in Icecrown as well).
Lana'thel has apparently shattered the blade and spread it across Azeroth, and it'll be up to us, the heroes, to locate the pieces and combine them with "a source of great power" to reforge the blade. Bornakk confirms that yes, the blade will be wieldable by players. Can't wield blades? No worries -- the preview hints that if you don't want the blade, the elves will take it back "in exchange for a usable weapon." So everybody ends up happy. Sounds epic and all, but what's with these old lore figures shattering all of their most powerful weapons? Can't they just put them in a safe deposit box or something? Do you really need to completely shatter it and spread the pieces around the world every single time? Come on now.
Update: Bornakk also says this will be available from 5-man dungeons -- it will still require some instance-running and dedication, but it won't be as huge as the Hand of Ragnaros or the old AQ Scepter questline.