Gabriel Maldonado wrote:
> 
> I think that writing an a-rate freq-cutoff version of csound filter should be a quite
> trivial job. Maybe I will do this in the future, because I'm curious about the sounds that
> can be produced. Are you sure that they are interesting?

Well, I do it in the analog realm all the time (the Multimoog, Octave
Cat, and Prophet 5 are a few of the synths that feature this ability). 
It is a very nice feature.  With lower amounts of resonance and a higher
cutoff frequency, the output resembles a "dirtier" version of the
input.  With a lower cutoff, nice ring-modulation effects can occur. 
With higher resonance, the sound is close to traditional FM.  The nice
thing about audio rate filter modulation is that any sound can be used
for the input, and the output will retain the original pitch of the
input sound (at least with filters that are not in self-oscillation). 
It works very well on drum sounds.