Add this to your commandline:
nouveau.perflvl_wr=7777

Will allow you to change the fan speed.

After a reboot, change fan speed by;
# echo 40 > /sys/class/drm/card0/device/pwm0

I put the following in the root crontab to monitor temperature:

* * * * * test `cat /sys/class/drm/card0/device/temp1_input` -gt 75000 && echo 100 > /sys/class/drm/card0/device/pwm0 || echo 10 > /sys/class/drm/card0/device/pwm0

mupuf on freenode#nouveau was of great help. You can find the nouveau project page here.