Dont assume the screensize is 320x240.
Yes that would be a bad assumption since the iPhone screensize has always been 320x480.
It doesn't take tablet to change that either. I expect future iPhone displays to increase the resolution even if the physical phone does not get bigger. Competitor phones are already at 360x640 and 480x854.
A smaller pixel size can be a competitive selling feature by providing a better user experience, especially as these devices are increasingly used for viewing detailed images such as maps or even rendering realistic 3D.