Apple's iAd service hasn't been as successful as the company hoped. While Apple has been able to keep iAd "premium" by keeping up ad costs and only bringing in high profile clients interested in putting out high quality advertisements, it hasn't actually made much money with the service, either for itself or developers who are trying run the ads.
Hopefully that will change now that Apple is bringing in a former Adobe VP named Todd Teresi to reinvigorate the iAd service and turn it back into the premiere ad and money generator that Steve Jobs hoped it would be. Teresi formerly oversaw Adobe's media solutions division, and he may either tweak the system so it's a little more appetizing for advertisers or bring more and bigger accounts on board so these ads can be served to developers.
As for the developers we've talked to, they're more than happy to keep serving iAds as long as they can also serve ads from other services. Competing ad services may not pay as much to developers, but at least they pay something -- many iAds never get shown simply because advertisers aren't willing to pay for them. Hopefully Teresi can fix that and get iAd back up to where Jobs originally wanted it to be.