KDDI, the Japanese carrier that launched the Fujitsu IS12T, has published an app in the Marketplace that enables users to migrate contact information from feature phones to their new Windows Phone handset. To do this developers will need system access to be able to manipulate base functionality.
To achieve this level of access, the contacts migration app uses the Microsoft.Phone.InteropServices.dll. The guys over at Nanapho.jp have published a short tutorial on how to carry out this native programming yourself. They do note however that the Marketplace doesn't allow this so it's only gravy for homebrew developers who wish to gain native access. Also, the registry is not included for the time being.