If you want ads on iPhone apps, you don't have to go to a new provider. The leading ad networks for mobile are also offering to deliver ads on them.
AdMob is offering $1Million in ad credits to developers if they sign up to use the AdMob platform. This clear effort at a landgrab could be higly compelling to a number of players, especially smaller players. The program offers up to 200 developers $5K each in credits. Certainly AdMob has deeper pockets than many of the startups, which could make a difference in the effort to attract the best developers quickly.
I haven't heard of any iPhone App speciifc activity from AdInfuse, but they have been at the forefront of iPhone advertising for awhile so I would imagine something will be publicized in the near future. Similarly, Third Screen has been quiet about their plans as well. Ditto Quattro and Ringleader.
Millenial Media announced an iPhone app ad offering on July 9 -- a bit ahead of some of the others in the space. CHeck out the press release here.