currently used ambient light sensor only detects light that is incidental, not directional. A combination of the light sensor, gyroscope and accelerometer could be a valid workaround, but the battery life necessary to run this would probably not make it worth doing.
Why would Apple even think about such a minute detail like shadows? Take a look at the video below which shows a design element in the upcoming iOS 6. The gyroscope is used to change the reflections on the Music app's radio buttons, depending on how the user is holding the device. By taking care of the small details, Apple endears itself to its millions of fans.
source: TheNextWeb, Gizmodo via AppleInsider