There are many iPhone ad networks. Perhaps the most popular is AdMob.
I've been using it for my application, iLaugh, for several months now and it's generating decent revenue (average $2000 per month on 175,000 daily ad requests). That low number is partly due to the fact that AdMob displays less than half of my ad requests as CPC ads, instead printing the rest with their sucky Download Exchange program.
What I would recommend, though, is going with AdWhirl, which is a solution that lets you dynamically change ad networks on-the-fly, put your custom ads or automatically balance between ad network to generate the most revenue.
If your app gains traction, you'll probably be better off negotiating a deal with one of the exclusive ad networks that veto your app based on traffic and merit and only accept the big guys. I've been accepted by the JumpTap network (implemented by Pinch Media) and am planning to use their ads as well as AdMob's in the next iteration of my app, which uses AdWhirl.
Oh, and I run a mailing list specifically dedicated to these kinds of questions / discussions: iphonesb.