Using a common light switch dimmer may not be the best solution for motor speed control. A dimmer is basically meant for a purely resistive load (lightbulb) and hooking it to an inductive load (motor) may cause problems from slight overheating to total meltdown of the motor and/or the dimmer. Some motors are more tollerant than others.
There is a fairly good description of why here:
http://www.act-solutions.com/kingery02.htm
It talks about X-10 home automation dimmers, but the standard "off the shelf" twist dimmers work the same way.
In short, if you can get away with it on your setup, ok, but if either the motor or controller starts to get hot, smoke, or outright burn up, it may be time to look for a different motor speed control solution.
Corey