Nobody associated with the show is allowed to say for sure, so we can only speculate at this point. Heigl has been rumored to be leaving all year because she wants to pursue a movie career, but whether or not Grey's releases her from her contract is another matter.
In my opinion, I think Heigl is done and Izzie will die. It seems like the writers have pretty much painted themselves into a corner with the character. The whole "dead Denny" crap made a LOT of people hate Izzie and her storyline, and now to have her come back from the brink of death would be just too much.
Plus, what would they do with Izzie if she does survive? Are we going to have to watch her go through rehab and learn to walk/talk/whatever again? Will she still be able to be a surgeon if she survives? Rehab for Izzie would be totally boring and have nothing to do with any of the other characters, and having her slip right back into her rotation after a few episodes of recovery time would just be too unbelievable (even for this show). So... nothing left to do except let her die. :(