Tue Jan 4 15:31:08 EST 2011
Nonlinear digital Moog VCF + bandlimited sawtooth.
Also mentioned in Stilson PhD is Antti Huovilainen: Nonlinear
digital implementation of the Moog ladder filter ().
Interesting derivation, but not such a surprising result.
However, the piecewize parabola is a nice trick! So I wonder if this
can be taken further one order by representing the remaining "bounce"
discontinuity by a 2nd order "brake" discontinuity and differentiating
twice, i.e. by using piecewize 3rd order polynomials:
(x-1) x (x+1)
Deriving once gives the piecewize parabola, deriving again gives the
sawtooth. The 3rd order polynomial's discontinuity is smoother so
should roll off faster (18dB/octave).
It shouldn't be too hard to implement in Pd either..
Indeed: differentiated parabolic (DPW) 
twice differentiated cubic (2DCW) 
Looks like the obvious extensions are already published.
V. Välimäki, J. Nam, J. O. Smith, and J. S. Abel, “Aliassuppressed
oscillators based on differentiated polynomial waveforms,” IEEE
Trans. Audio, Speech, Language Processing, vol. 18, no. 4,
pp. 786–798, May 2010.