The photocell is the same one that Adafruit sells or at least it is very similar and is the first datasheet on the learn site for photoresistors.
http://learn.adafruit.com/photocells
"These stats are for the photocell in the Adafruit shop which is very much like the PDV-P8001 "
The DigiKey link is
http://www.digikey.com/product-detail/e ... -ND/480602
The 5,6K resistor value is the result of John and I discussing what value would be best. It was decided that a value between 10K and 1K would probably be best all around. As John mentions if you know the environment then you can match the resistor closer the the light level found in the enviroment. If you don't know the environment the clock may be in or if the clock will go from a dark area to a bright area, then I would recommend something around 5.6K.
The 5.6K is what is listed for the resistor in the REV. D xams board and the photoresistor PDV-P8001 is also listed.
http://www.digikey.com/product-detail/e ... BK-ND/2243
and
http://www.digikey.com/product-detail/e ... -ND/480602
Of course, your free to order from your favorite place.
On the lockbits.
I had a case where the chip erase during programming did not appear to erase the lock bits which caused the chip not to program, and I had to try several times to the point where I thought I did something wrong and had bricked the chip. I was thinking that if someone was interested in maybe trying different firmwares then it would be best to not set the lock bits, just in case they don't erase properly or reset properly. That would remove a possible obstacle. If your not planning to try different firmwares, are giving the clock as a gift, or are aware that a chip might give some trouble erasing then I would recommend setting the lock bits.
One curious thing that may support detting lock bits. I pulled my UNO R3 chip the other day, set it aside and when I put it back in it (the bootloader) was corrupted. So maybe setting the lockbits would help prevent that from happening because of static.