Cory's idea is basically sound. I like to use a system timer (simple timer interrupt)and decrement a "step count" inside the interrupt. The value loaded into the "step count" is inversley proportional to the speed.
You just need a way to change the value of the "step count" (like an external dip switch or serial command) and move to the next step when the count depletes.
As far as performance goes, I've used the 8051 for several stepper motor applications (and there are many I haven't attempted) and the 8051 has always done well. Personally, I've never selected a stepper motor for its speed, only for its precision.
But consider this:
You can easily get a system timer of 50us with a 12MHz clock. Taking into account overhead that is easily done.
If I then step at a 50us rate (approximatley) thats 1sec/50us = 20,000 steps per second. If I use a stepper motor with 200 steps per rev I then have 20,000 steps/s / 200 steps/rev = 100 revs per second which is 6,000 RPM.
Okay, that isn't good for a dental drill but it is pretty quick. Of course you have to deal with the load at that speed to see if it is truly feasible. The reality is, I have always been able to out step (change steps faster than the motor can spin) my stepping motors with an 8051. Yes, I use good ones.
You should have no trouble changing speeds if you follow this approach.
Allen