Well, you don't have the option to opt-out. You have to opt-in - unless you explicitly say "I want to see ads" in your user options, you will never see them. ;-)
I find it an important difference. "opt out" is the spam model - "we bombard you with ads, you can say no, let's see if you can say no faster than we multiply". Opt-in is the model for adults - "unless I know you want it, I will assume that you don't".