Are you serious?
"When is ad-blocking ethical?"
Er, that's easy. 100% of the time. I will decide - as far as is practical, when I do not wish to see an advert. If I can skip adverts on the PVR, then I will. Same principle for adverts on the web. If I am driving along and am advertised to on a billboard then there is not much I can do about that, but that's life.
To all marketing types out there: Suck it up and go get a proper job. Stop promoting consumerism, it is a fundamentally flawed proposition.