I feel compelled to reply to these messages. My electronics is a little rusty, so please bear with me.
To NotSilentYet: Your circuit is an emitter-follower design. The base-emitter junction voltage of your transistor is around 0.6VDC at saturation. With your load (fan) in the emitter lead, the combined current through the tansistor could cause the base-emitter voltage to increase past 0.6VDC and damage the transistor. Hence no fan operation.
A slight circuit redesign should be to relocate the load (fan) to the collector lead of the transistor, a series base resistor to limit the base-emitter junction current (voltage) and tie the emitter to gnd.
To Clamrade: A rheostat is better suited for direct control of a fan, and a 1/4-watt potentiometer will burn up if the fan draws 68mA. Simple ohms law tells us to calculate wattage, we need to multiply the voltage by the current. I.E. 12V X 0.068A (68mA) = 0.816-watts. I would recomend using at least a 2-watt-rated potentiometer or rheostat.
Please don't get me wrong. If you are a techy & handy with a soldering iron, build up a transistorized fan controller. It may give you the results you require and provide you with a circuit that you built yourself.
If you don't have the electronics wherewithall, maybe use a rheostat.