Special thanks to GC for pointing me in the right direction regarding AC motor control using triacs.  Rooting about on the web, I found an excellent Word document describing AC power control using a microcontroller.  Basically, the technique is the same as using PWM using DC motors, except that timing becomes more critical.  Power is turned off when the voltage crosses the origin, for whatever the PWM period is, and is turned back on for the remainder of the cycle.