Check out the Wrathgate Cinematic at 2:14
http://www.youtube.com/watch?v=WrMa_sU1Nnk
It looks like Arthas Absorbs Saurfang JRs soul into hes sword. And the Frostmourne room really do look like some kind of prison due to the spectral chains in the sky. It would make sense since Bolvar returns in ICC.
I dont think Saurfang is an entierly new boss, i just think hes a part of the whole lich king encounter. And i dont mean Saurfang will be the only soul in the frostmourne room. Hes hordes version of Bolvar, and Bolvar got a big role in ICC. Im sure there will be plenty of other angry spirits there too.
Errant said that King Terenas Menethil II, Arthas father, also might appear in the frostmourne room, as he died by Arthas sword. That would explain the new model of him.