I had never heard of these before, until I spotted this on a random blog when I was searching for something else.
They are made with a #10-12 steel hook and three strands of sewing thread. One strand at a time is swapped every few rows for the graduated color effect. I've never made a doily before, but my first reaction upon seeing this was "Wow!" :shock:
A little more searching resulted in finding this photo (Doily #2) and this book on painted doilies that apparently is no longer available. It could also be done with embroidery floss (with the 6 strands separated).
Anyway, I thought this was worth sharing. I might have to try this out some day.