Feet sliding left / right? Here's why!
I mistakenly animated the feet motion along the Local (or Normal) axis rather than the Global! Because the foot bones in the rig are not straight in front of the body, but are posed at an angle by default, this results in the feet appearing to slide left / right if transformed along their local axis. Unfortunately, you can't tell this when animating from side view... it only becomes apparent when you switch to the front. So... back to the drawing board. Hope this helps someone else who thought their version of Blender was broken, since the foot bones' X locations appeared to be fixed at 0, and yet there was all this "sliding"! :-)