Thanks, everyone for the explanation. It's just that I'm no bike mechanic, and I'm never able to shake the feeling that I might have done something stupid to cause X problem. See, I'm a threaded headset guy. And the first time I was swapping out the stem, I did *not* expect the fork to tumble out. ("Holy ****! I can see the ball bearings!") So I put it back as best I could, hoping everything was seated correctly. When the 'index steering' came up a month later I thought I may have been at fault.
Then again, the bike's three years old, and I've owned it for three months. So I'm hoping that this is just the end of a normal headset life cycle as opposed to another 'the beef'-effup. That'd make me feel better.