Unipolar and Bipolar Stepper motors
You can drive a unipolar stepper with a bipolar stepper circuit-- just ignore the center taps.  In fact, it should be more efficient to drive a unipolar stepper this way.  The only reason (that I know of) to drive a unipolar stepper in the traditional way is to simplify the hardware; if you've already got a pair of H-bridges, you may as well use them.