ESRI has released the ArcGIS API for iOS:
The API is designed to use Web services available from ArcGIS Server and ArcGIS Online. Users of the platform can access dynamic, tiled, and image map services; overlay graphics; search and identify features; locate addresses; collect and update data; and perform GIS analysis. The API enables organizations to build and deploy solutions that extend to a wider market via the App Store. Developers and Esri partners can also use the API to create applications for both external and internal use.
The API can be downloaded for free from Esri’s ArcGIS Resource Center.
Esri Releases ArcGIS API for iOS (press release)
ArcGIS API for iOS – information page from Esri