According to this post on the EU forums (hope you've brushed up on your... German?), the six raids assaulted three cities simultaneously and killed the Prophet Velen in the Exodar; Tyrande Whisperwind and Arch Druid Fandral Staghelm in Darnassus; and Archbishop Benedictus, as well as Highlord Bolvar Fordragon in Stormwind. After laying waste to the three capital cities (or their leaders, at least), the entire lag-inducing, server-crashing set of raids stampeded to Ironforge and let King Magni Bronzebeard and High Tinker Mekkatorque in on the fun.
Although ironically a PvE endeavor because faction bosses themselves are NPCs, this is thematically and lore-wise the stuff of my PvP dreams. I've always daydreamed about exactly this -- laying siege to all capital cities at once. Kudos to the Horde players of Eredar (EU) for preparing a nice Christmas present for the Alliance. The video is a hefty 245 MB download, but if you have the free time, it's well worth the watch.