I think if Izzie were really dead, they would have involved Denny somehow, no?
Shonda Rimes is really strict with what she lets the cast say re spoilers, no-one is allowed to say anything they haven't been allowed to say about future episodes so it's probably true what Justin said. Although it could be that not all cast members last the whole season or are in flashbacks or something. It definately seemed like they were phasing out George for awhile. Even when Katherine Heigl asked to be 'written light' in the 4th season because she was doing movies, she still had a storyline, let alone a presence. George definately hasn't had the former this season and has barely had the latter, so obviously something is happening there.