The technique of using an LED as a light sensor does work, I have done it my self:-http://www.thebox.myzen.co.uk/Workshop/LED_Sensing.html
However it is dependent on the LED and also dependent on you actually shining some light on the LED to detect.
I should be considered a novelty and not a serious reliable sensor. There is nothing on the UNO that will stop this technique from working so if you are having trouble then it is either some error you have made or the type of LED you are using.
For me A0 is not declared and it returns an error at compile time.
Under version 20 and beyond A0 is a predefined constant.