Here's a list of the materials I used:
* Arduino Uno (running on Mac OS X 10.7.4 Intel)
* USB cable
* Super-cheap earbud style headphones (from a plane flight, I think)
* standard small photocell
* 10K Ohm resistor
* Some wire
* Soldering gun with solder
Step 1: Hack the headphones
First, once you cut the wires and strip off some insulation, you should see 4 wires, 2 from each side. Each pair forms a circuit with a single headphone. So we can effectively think of each pair as containing a + wire (usually colored red or blue) and a ground wire (usually copper colored). We can either just use 1 of the headphones (and thus one of the wire pairs), or we can combine the wires to use both, thereby achieving twice the fun. To do this, we will want to solder the + wire from each pair together to one jumper wire, and the ground wire from each pair to the other jumper wire. See detail photo of headphone hack above.
Second, beneath the obvious rubber insulation, there is often a mostly invisible resin layer coating the 4 individual wires. The easiest way to get it off is to heat it with a lighter until the wires appear to burn briefly. Note that after this is done, the colors can be very difficult to discern, so it is a good idea to leave a section unburned that you can use to identify them. I also use a wet cloth to wipe of some of the residue from the burnt resin to improve conduction.
For the actual soldering, I have very primitive equipment, so I just twist the headphone wires together and around the jumper cable and then drip some solder on that.