Well somehow they managed to figure out where I’d been if it wasn’t MAC address. Well enough to show it as a choice when I visited Google Maps. Maybe it’s getting too late at night.
Your browser provides a lot information about your system, for example the browser type and version, the operating system type and version (including major service packs), the browser plug-ins and their versions, the screen resolution and DPI of your display. You add all that together and it makes a pretty good signature. People install different plug-ins (Adobe Reader, Flash, etc.) for different needs.
When you log into Youtube, Gmail or any other Google service they have who you are with your signature. So when the signature moves across different networks, they can track you without cookies or logging into anything. All without your MAC address.
IPv6 is different. It normally uses your MAC address as part of your IP address. Every MAC address is unique making each IP address unique. Therefore you can be tracked everywhere you go directly including everything you view online. I find IPv6 to be rather disturbing for that reason.