I'm pretty sure Arthas will be at the end of a 10/25 man icecrown citadel raid because at Blizcon the guys at the raid panel said so, they showed first the icecrown raid with Arthas on the Top Floor, and then went on to the icecrown citadel dungeons, Whether he'll 'poof' away or not im not sure.
-H

Ps This is where they say it: http://www.youtube.com/watch?v=cy9zdoboAPo