Evidently, you're asking for a service that few if any of us have ever contracted.
I noticed that Dan Benjamin (5by5.tv) has sourcebits.com as a sponsor on The Talk Show. They show an impressive portfolio, and I trust that Dan has done his due diligence on this advertiser. Have you looked at their service?
Update: Someone just mentioned the Stanford course today. From your initial question, I presume you didn't want to code the app in-house. If you are interested in going that way, the courses, books, and services from the [Big Nerd Ranch][1] are highly recommended.
Update 2: [This forum][2] is a place for those interested in contract or hiring iOS developers for their projects. You may learn some things by looking there over what people offer and which offers get responses.
responses. I see that some shops offering services hang a shingle in the forum.
[1]: http://bignerdranch.com
[2]: http://www.iphonedevsdk.com/forum/iphone-sdk-development-jobs/