We've got a few months of speculation to get through before the next season of Grey's Anatomy rolls around. I imagine that official news regarding who is staying and who is going will be a long way off, and if Shonda & Co. manage to get their way, we might not know anything at all until the season six premiere.
So all we've got in the meantime is each other and all of our crazy theories as to what might happen. I'm leaning toward the theory that Izzie survives and George walks into the white light. I don't have any insider info to go by, so this is mere fan speculation. But I think that Izzie surviving makes the most sense. I say this not because Katherine Heigl said she'd stay with the show, but because storyline wise, I think her surviving is important.