I downloaded and built from the CVS and that solved the segfault. Now I get:
hbarta@toonsinator:~/nvclock-cvs/nvclock$ nvclock -F 80
Error: Your adjustment of the fanspeed isn't supported on your type of videocard!
So I guess the fan is not adjustable.
I also built nvclock_gtk which puts up a nice dislay with no mention of the fan speed. I wonder if I can underclock and safely kill the fan.