I never thought I'd see this happen, but it's official.
I earned more in December from Amazon ($438) then I did from AdSense ($427).
That's my highest month ever for Amazon, of course, and one of my lowest in recent times for AdSense, partly due to the hacking of my site, but also due to December, which generally isn't a good month for me for AdSense...
I have to say that much as like AdSense, with Amazon I can see the connection between my efforts and results. If I build good content for Amazon, my sales go up. It's all a bit more mysterious with AdSense!