William sent in this page detailing how to replace the bulbs of Christmas light strings with LEDs - I begin by cutting the light sockets off the string about three inches from the socket and I store them in a box. There is usually a wire that goes from one end of the string to the other (two such wires if you're lucky to have a string that has an electrical socket at the end of the string), which I wind onto a spool. Sometimes you can find wire the right shade of green wire at electronics stores. I find that 22AWG size stranded wire (make sure it is not solid) works the best. Though LED christmas lights can now be purchased relatively cheaply, this could make for a neat holiday project - LED Christmas lights
http://blog.makezine.com/archive/2008/12/converting_to_holiday_lig.html