There is an article in the week's Adweek called "Do Web Ads Lack Credibility?" The article cited a study that claimed when it comes to "trust," banner and search ads score significantly worse than either magazine or television ads. You can read the entire article here:
I'm not at all surprised that web ads lack credibility. Because, once again, ad agencies and media companies (not to mention web sites themselves) who have failed to police the properties they support or own. How many times have you clicked on a banner, only to have it not allow you to click off it? Or what about those asinine ads with the people dancing in the background (for no apparent reason other than to catch your eye)? Or even "personal ads" with impossibly beautiful people who are looking for you you you?
Agencies, clients, etc. have allowed such clutter. Such ads denigrate everything else around them. Yes, other mediums accept ads from shitty companies. But on TV or print those ads are usually relegated to late night or small space. Therefore they are "outweighed" by more credible messages. In more established media, you tend not to see ads for the University of Plumbing Arts butted up against something from Verizon.