IanTurner - 10:54 am on Mar 10, 2010 (gmt 0)

People will block ads when those ads become a nuisance to them while browsing.

Ad blockers become popular when the ads reach new levels of nuisance value - such as when pop-up ads became commonplace. I think the current upsurge in the popularity of ad blockers is because of the level of ads with sound and video both of which cause more irritation than flat text or non-moving image ads.