I was playing on the newest snapshot this morning (12w27a), and I noticed after I had broken some blocks above me into the underside of a lake bed, that the water flowing in gave me hit damage. I validated that I was not drowning because I never went below about 3/4 air bubbles. After a little testing, I could reproduce it consistently by placing a bucket of water in front of me and letting the flowing water push me against the wall. I would receive rapid 1/2-heart hit damage as long as I was against the wall.
Has anyone else ever seen this before? Is this a new feature/bug in MineCraft? I searched the talk pages on all relevant subjects and the change logs for all previous versions and cannot find this behavior documented anywhere.