The story is that Ensidia made use of Saronite Bombs to "bypass The Lich King fight mechanics" (Saronite Bombs and similar items were disabled in a hotfix last night). In addition to the temporary ban, all items and achievements they gained from downing Arthas have been revoked. Before the ban (but after the hotfix), Ensidia put up a post claiming that they didn't think the bombs were an exploit; Blizzard obviously isn't buying it.
Meanwhile, Muqq, the Ensidia player who posted about the ban, has taken this as an opportunity to quit WoW (and rant a bit at Blizzard about "half-assed encounters"), saying "to ban people when they do not know what's causing the bugs is just a [expletive] joke."
Update: It's worth a mention that the language Muqq used at the end of his post is identical (save places and names) to this post by Tigole (scroll to the bottom -- it's the last thing on the page), written of EverQuest in 2002. Be warned, neither of these are safe for work.