The "about Alexa" page on their site is pretty generic, basically saying that they use info gathered from the Alexa toolbar. I haven't seen anything that actually explain what they factor into their results and how, but in all honesty sometimes I have a "blind" moment so if they've got a page I can't find that's entirely possible!
BTW, I don't mind having my chops busted if I deserve it.
Great info Opti, thanks!