I believe the problem started late last October, during a backpacking trip. I was climbing up a steep section of the trail over some boulders, when my left foot slipped. I reached out with my left arm to break my fall, and felt a sharp pain in my shoulder. It didn't feel too bad, though, and I continued my hike without any other problems. Later I learned that the medical community calls this a "FOOSH" - a "fall on an outstretched hand".
Over the next few weeks, I began having some shoulder pain during my weight-lifting workouts. I keep an exercise journal, and the first time I mention the pain was on November 11, 2009 - "left shoulder hinky".
Also, I recall that moving my left arm behind me to slip on a backpack became painful in the weeks after the fall. I adapted by putting on the left side first, so I wouldn't have to reach back with my left arm.