I've been working on it for years, and recently pretty much gone into a cave to try to finish it, and, at long last, it's nearly ready. I finally have a reasonably complete draft of my PhD thesis on interpolating splines.
It's quite technical, and I'm not expecting too many people to follow along. Even so, I hope it gets across some of my motivation. Even though I hope the splines are useful in many applications, my primary reason for doing the work is to have a better tool for creating fonts. Also, I hope it gets across how deeply I have gone into the literature and theory.
The draft (in a compact format, not the actual thesis template) is here: http://levien.com/phd/frame.pdf
If anyone is inclined do detailed critique, chapters 1, 5, 6, 7, 8, and 10 are in near-final form. There's some reorganizing to be done of the elastica material, and integrating the results on the generator curve from the CAD paper. But overall I think it's pretty coherent. I'm excited!
Thanks to people here for expressing interest in the work, and responding to various queries and surveys I've thrown out over the years.