Use Case: I plan on marketing iOS apps that I own & operate. I plan on marketing these apps on FaceBook and Google. I want to understand my funnel conversion by source.
If it's a paid app, you can use the iTunes LinkShare affiliate program:
How to track marketing efforts for iPhone apps on the App Store?
http://www.quora.com/How-can-I-measure-app-download-conversion-in-iTunes-appstore
If it's not a paid app, you're out of luck without a heavy 3rd-party tracking system tracking click origin, and matching that to your internal app install ID process. Painful.