There is a forum thread about this linked in the playground, but I don't have enough language skills to read much of it, so I have made a new example for this. http://www.arduino.cc/playground/Learning/LEDSensor
In summary: It is quite easy and surprisingly accurate. You can even, as the example shows, use the same LED for light sensing and light emitting. This example charges the junction then times how long it takes the charge to migrate across.
You can also do a +5--NP--AnalogIn--10Mohm--ground arrangement and use an analog in, but it isn't as sensistive, though it does get very slow in the dark. And you might want bigger than a 10Mohm, I didn't have any big resistors handy and was just using my scope probe as a load.