I had this problem before -- I accidentally set my iPhone to "sync-only" rather than drag-n-drop. (I forget what the actual setting is called.)
And when I say that I did it accidentally, I mean, I don't know what I did to do it, and it happened during an upgrade of iTunes, so I really think it's a side-effect, but the solution is the simple "set it back" one, as if you had done it yourself accidentally.