In depth foot exam, lengthy ultrasound, another look at yesterday's x-rays, and then some additional real-time x-rays at the podiatrist's office this morning.  Final diagnosis is a stress fracture, thankfully minor.  The pain (and resulting limp) is not as bad as it was originally several weeks ago, so it seems to be healing itself.  However, I've been instructed to stay off of it as much as possible this next month, and (of course) no weight-bearing exercises at the gym, no planks or squats at home.  Obviously I'm also off of my cardio (elliptical/steps/treadmill) for the duration.  And I was making so much progress.