hey, just wanted to say thanks again for your help. with this function, i was able to get non-blocking stepper control working!!! if you've ever used the built-in stepper library for arduino, you know that its blocking... ie you either have to call each step manually, or just deal with waiting until the steps are done to do more stuff.
using this function, i made a class so that you simply do if(stepper.canStep()) stepper.step(); and the stepper will step at the appropriate time, at the appropriate RPM. this is nice because you can do things like control multiple steppers independently, etc.
more info, video, and code in blog post: http://builders.reprap.org/2007/11/non-blocking-stepper-control.html