The stem didn't fail. It was the locking mechanism that wasn't properly engaged. It had seemed a bit loose the night before, so I adjusted it. Now that I think of it, after the readjustment I didn't get the firm "locking" feeling (sort of a muffled click) that you normally get -- but it felt tight, so I didn't think much of it. Now I know that the click is all-important! Also, I have been riding without that little plastic ring that provides a backup to the stem lock mechanism (broke off within weeks of the bike's purchase). It's high time I got that replaced...

To give a bit more information, this is an older Boardwalk -- I'm not sure if the stem / locking mechanism has been changed on the Boardwalk for this year. Anyway, it's a different design from that on my parents' Speed P8s.

It wasn't actually too scary, but I certainly let out a few choice words as I went down. I had been in my hard-core bike commuter mode -- messenger bag and all, sprinting from the light -- so I suppose I had it coming. Anyway, had a good laugh about it with the traffic guard!