moveable frets

Hi. I was wondering if there are any manufacturers of guitars which have
movable frets (much like a sitar or even viola da gamba in nature). Or,
if anyone knows of a site or book which might explain some technique in
making a fretboard that has movable frets (and is still as playable as a
fixed-fret guitar)

Thanks,

-Shaun Van Ault

Oberlin College
3rd-year composition major