Above the threshold voltage the drain-source resistance is a non-linear function of gate voltage. See **broken link removed** for more info. It refers to J-FETs, but the same is generally true for MOSFETs. Edit: The reference does show a technique for making the resistance linear.
This will also vary between transistors, depending upon both the threshold voltage and gain of a particular transistor. Thus ii you want to use a MOSFET as a variable resistor, it is usually in some sort of circuit where the effect of the variation is minimized (by having it in a negative feedback loop for example).