This question comes up from time to time. It actually has to do with the type of motor it is. It sounds like you're considering an 'induction' motor. This type of motor operates a certain way.
A router motor is what's called a "universal" motor. So named, I suspect, becasue they're able to run both on AC or DC power. They operate using brushes that need to be periodically replaced.
The speed controllers are designed for use on universal motors only. That is, the speed controllers that you can buy for under 35 bucks that are used for routers. They make speed controllers for induction motors, but they're quite expensive.
Basically the induction motor's speed is based on two factors: The frequency of the AC power (60hz, in the US) and the number of distinct windings. The latter is almost always 2 or 4 windings and is obviously not trivial to change. The frequency of the power is adjustable, but takes some fairly spendy circuitry to do so.
The universal motor's speed relies mostly on the amperage being supplied to it. These speed controllers have a meaty rheostat that that adjusts this amperage. But, it also reduces the operating power of the motor as well.
That's basically why it won't work, as far as I understand it. Hopefully someone who knows more than I do will come along and fill in any details I've missed.
Last edited by Jason Beam; 06-13-2007 at 04:05 PM.