Although several realms are still under extended maintenance in preparation for Cataclysm, Patch 3.3.5's Ruby Sanctum is currently open for business. Players can enter the instance at this time and kill the trash and minibosses, and Halion is able to be engaged. We've not yet received word on whether or not he's killable, or whether anyone has killed him. It's likely that this was unintentional.
We'll keep you posted, though. Now would probably be a good time to brush up on the dungeon's final boss with WoW.com's Guide to Halion!
Update 6:29 p.m. EDT: The opening of Ruby Sanctum might be a bug, but nonetheless, it's up and running. We have staff who are running it both on normal and heroic mode on the live servers. Blizzard is being coy about it, so who knows ... either way, the boss is up and can be engaged right now.
Update 6:39 p.m. EDT: Nethaera confirms that this isn't a bug: Ruby Sanctum is open, despite several battlegroups being down for another day.