Englische Minepedia has written
Ghasts
Notch commented on his Twitter, saying he "made the most unfair mob ever even more unfair," and that we "will hate the Ghast". He also stated that it is currently just a 4 meter (4 block) grey cube, is not invisible, and since "fire works surprisingly well [at killing it] considering it lives in the hell dimension," has been made immune to fire.
The Ghast will appear in Hell in the Halloween Update.