dont get me wrong, f/2.4 is a great change from f/2.8, but i think it should be looked at as a whole. the combination of an f/2.4 with the sharper 8 megapixel camera, CMOS sensor, AND noise reduction....THAT is an upgrade. they all benefit from each other.
think of it this way...you can have a 70-200 f/2.8 on a cropped sensor. yeah it'll be cool and will work....but not nearly as amazing as a 70/200 f/2.8 with vibration reduction on a 5D Mark II or D700 full frame sensor.
think of it this way...you can have a 70-200 f/2.8 on a cropped sensor. yeah it'll be cool and will work....but not nearly as amazing as a 70/200 f/2.8 with vibration reduction on a 5D Mark II or D700 full frame sensor.