There is an interesting discussion on Slashdot, reflecting what Bennie Smith (of Double Click) has previously said, about the end of free Internet contents when advertising blocking get included in web browsers as standard feature.
The reason I say "opportunity cost" is, advertising only brings in revenue if the user makes a click-through, as most ad agencies are paying on a click through rate. With this point of view, I can safely say that I have never and probably will not steal any revenue from an ad-serving content site, as I never click on a banner ad. But I guess each individual varies. Banner ads survive - because they bring in enough revenue to both the advertiser and the content provider - because people do click through them and make purchase.
Do you click-though banner ads and make purchase? Do you block them using a proxy or browser filter? What's your view on the ethics of blocking the ads? Would you spend that much time on-line, if suddenly all the worth-reading content sites now demand subscription? That's to be discussed.