TO control multiple steppers it would be easiest to have a separate drive for each stepper motor. A typical driver has 2 inputs - Step and Direction (and sometimes an Enable) This would mean each stepper would require 2 Arduino outputs (or if multiple motors move the same you could double up on some of them) Depending on your motors, you can find some simple small drivers or you can get some better commercial units.
If you want to build your own here is an idea of how to do it with Arduino tyoe tech... http://arduino.cc/forum/index.php/topic,84809.0.html
This is something I hope to finish up on this winter - Uses an ATtiny2313 which accepts step and direction inputs and outputs the 4 phase signals to drive 4 MOSFETs to run a unipolar stepper. By using a driver the code in the main arduino gets simpler and mans you can actually do more.