Okay, if you saw the 5th Season Finale last night, then you know there is a major cliffhanger. Here is my opinion. (Warning: SPOILER)
We learn that Georgia is "John Doe" who was poorly mangled and burned when he pushed a girl out of the way of a bus and was hit himself. He writes "007" on Meredith's palm post-surgery #1. Meredith (in a great acting moment) freaks out and says, "Oh my God, it's GEORGE!" George had just signed up to join the Army as a Doctor earlier that day.
Now, knowing that both TR Knight and Katherine Heigl wanted to leave the show, I think that George is definitely dead. Chances are, at the beginning of the next season, it will start sad and somber, people dressed in black for George (and maybe Izzie's) funeral. How do I know this?
Did you see George at the end? Did you see the massive, serious, disfiguring burn on the side of his face? He's not coming back as Two Face. I'm pretty sure he's done. Even Mark Sloan couldn't repair his face.
Izzie. Well I don't know. I think they might drag Izzie's character out a little longer. After all, at the end, George was already off the elevator (why was he wearing Dress Blues, or were they Class As? I can't remember. He only signed paperwork for the Army.) Izzie, however, gives George (and past George) a look like, "do I want to go? should I go?" and she never gets off the elevator.
I think she'll come back next season for at least one or two episodes on life support. While she's on life support, I have a feeling we'll see an alter world in which Denny is there and George, and they'll talk to her (kind of like when Meredith "died").
Then, I think Alex will have to make the decision to "pull the plug."