just want to say totally enjoyed reading bella's blog
i think about this topic very often cuz i "can't" wave while riding, not cuz i'm "ultra self centered". my first time ever on the bike (no tricycles, no pretty pink bike with baskets) was only two short years ago. learning as an adult means there isn't the same sense of balance or control, thus i ride very carefully with BOTH hands on the handlebar ALL the time! instead of waving, i would lift my fingers (except for the thumb and index) or i would smile, but then i have oriental eyes, so when i smile, my eyes disappears and can't see the road, which is even more dangerous!
anyway, just thought i threw in the 2 cents that sometimes it is not always the case. sometimes there are other explanations... give others some benefit of doubt.