24" 1920x1200 screens are over, they all became 22.x" and 21.5" 1080p screens so the manufacturers can get more screens out of a production run. With the increased pixel density of the 21.5 and 27 iMacs plus the MacBook Airs models, plus iPhone 4, it's not likely that there will be another 24" iMac.
How about 23.5" screens? They seem to be very popular all-in-one computers today.
That could also mean that the screen size is cheaper since it is mass produced.