Ethical is a strange word to use when discussing the use of AdBlock. It should be unanimous that it is unethical to take away the income of sites/personalities that go out of their way to provide content for their users/fans with no extra charge (bar advertisements.) However, that's not why I use the extension, I use it mainly because it eliminates tacky flash banners and pre-video advertisements on YouTube. I don't have it on all the time though, I tell AdBlock to stop running on sites like Giant Bomb and various YouTube users etc. because I like the site enough to sacrifice convenience in order to support them.
So in answer to your question, no, it's not really 'ethical' but I don't think that's the right word to use for this discussion.
@ch3burashka: I've had the same thought whilst using torrents (legal or otherwise.) Thankfully though I haven't received any kind of 'cease and desist' letters from my ISP and I hope it stays that way.