Today I finally worked out all the bugs in the IK and servo driver software.  This now allows me to set any leg at an absolute x,y,z co-ordinate (within reason, of course... there are physical limits to how far each leg can move).  To demonstrate this, I have a video of Stubby's body moving along the Y axis without the feet moving (or more accurately, all of the feet are moving along the same axis at the same rate, essentially moving the body).

I think the next step (no pun intended) will be to create another abstraction which will convert body commands (move forward, turn, translate body on X axis, etc) into a series of leg commands.  This may end up being another IK-style library, or it may be something like an animation (just a series of steps)... I am not really sure of the best way to go about doing this.  If anyone has a recommendation, please let me know in the comments!