So far, not a very helpful thread...
I just got started with this stuff around June (2014) and so I consider myself a big "newbie". I have been on the same quest for some time now - going in circles it seems to get my inverted pendulum robot (long way of saying "two wheeled robot" to smoothly move using 2 stepper motors.
Thought I had made a breakthrough when I ran across the "accelStepper.h" library. while it has many nice functions to accellerate and run the motors, I have yet to be able to code (or find code) that will allow me to change the speed while the motors are still moving. this is a big drawback, but one that I'm hoping someone will chime in with sometime over the next few months. i have my Accelerometer & Gyro working in harmony, just need a way to control the Steppers smoothly. If not, i'll move on to DC motors that can be controlled with PWM.
anyway - you may have better luck than i with the accelStepper.h library. look around this site - lots of good documentation on the functions:
http://www.airspayce.com/mikem/arduino/AccelStepper/
Russ from Coral Springs, Fl