I just posted an interesting discovery on the FreshBooks blog – the post includes a really useful tip for anyone building a web app.
Check out the article, and consider what the reduced conversion rate implies about users and their disdain for marketing offers in their user experience. Paul Kedrosky wrote this on eBay’s decision to offer Goolge Adsense ads on their site:
“any high-traffic application that does not also run ads is passing up material revenues, and its shareholders should take it to task.”
To which I replied:
“I’d like to see more data on this. While you are probably right, when you run ads you give up realestate and make usability trade offs that may have more of a long impact on your numbers than may be apparent at first blush. eBay built eBay didn’t they? Part of the appeal for users has been lack of ads. Isn’t lack of ads a community first approach and therefore a good thing long term? I’d say yes, but again, I’d like to see some real numbers on this…and let’s not forget when google runs ads in their free apps, they have no middle man therefore their margins are not comparable (read HIGHER) than other apps who run Google’s ads….”
How do design distractions affect your bottom line over time…hard to calculate, but worth considering.
Michael McDerment 
This reminds me of some information I say earlier about ugly websites producing more ad revenuw:
http://www.site-reference.com/articles/Website-Development/The-Surprising-Truth-About-Ugly-Websites.html
Also some sites try to confuse people into clicking links as much as possible. Which is something you usually have to put up with if you want something free. Like this site:
http://www.eggdisk.com/
ugly as heck but does give you 5 gigs of online storage for free.
I enjoy ebay not having ads, however I’m pretty sure they surely would increase income by using them. Perferable they would only leave the ads on search pages or on the lists page. Not on individual postings, if they did thats when i wouldn’t sell there anymore.