Aside from whether Apple matters (whoops!), the biggest thing I've changed my mind about is climate change. There was no one thing that convinced me to flip from wait-and-see to the-time-for-debate-is-over. Instead, there were three things, which combined for me in early 2006. There was, of course, the scientific evidence, which kept getting more persuasive. There was also economics, and the recognition that moving to alternative, sustainable energy was going to be cheaper over the long run as oil got more expensive. And finally there was geopolitics, with ample evidence of how top-down oil riches destabilized a region and then the world. No one reason was enough to win me over to total energy regime change, but together they seemed win-win-win.
Now I see the entire energy and environmental picture through a carbon lens. It's very clarifying. Put CO2 above everything else, and suddenly you can make reasonable economic calculations about risks and benefits, without getting caught up in the knotty politics of full-spectrum environmentalism. I was a climate skeptic and now I'm a carbon zealot. I seem to annoy traditional environmentalists just as much, but I like to think that I've moved from behind to in front.