Interesting circuit mike, I was thinking of using a micro rather than discretes, that said I might be able to borrow some of your ideas from that for the feedback and firing circuits.
I've never seen anything like that before, I can see your filtering the mains into a triangle and then using a comparator to compare that with a dc ref from the pot, working out how you got feedback is gonna take a bit more staring at that and more coffee.
Bits: I found a company that sells a controller for washing machine motors aimed at the model making industry, as a cheap source of power, I think they use the tda 1085, like I said I want a pic, however their instruction manual says that the tacho is a freq out, whatsmore you can solder on more poles to increase the o/p freq, interesting, if I were to use timer0 or timer 1 with ext clock I'd be able to get a better resolution and lower speed without loads of code.
Edit: looks like I was wrong, the motor circuit looks as though its part of the oscillator.