I have on occasion controlled the fan speeds by going to:

Code:
/sys/devices/platform/applesmc.768
There you will find several files referring to the fans.

Such as:

fan1_min
fan2_min
fan3_min
and many others.

Make a note first of the values in fan1_max, fan2_max and fan3_max so you know the max speed for each fan.

Then you can control them by doing:

Code:
sudo nano fan1_min
And entering a desired value.

fan1 is for the GPU I believe.

I have not researched the other fans.