I developed a fairly simple circuit designed to convert any brushless DC Fan into a variable speed unit. The details can be found on my webpage, http://www.rpi.edu/~hicksm in the section named "PWM Fan Speed Controller. The unit costs about $10 to built, provided you have some basic electronics tools. Feel free to e-mail me with any questions.
The use of a potentiometer for fan control, in my opinion, is dubious at best. When one uses a pot, the energy that would be used by the fan is dissipated as heat by the pot; this is defeatind the point. Also, the RS pots are typically 1/4W units. To drop 5V at 100mA with the pot would dissipate 0.5W - twice the rated spec.