public interface NativeMobileApps
Both Titanium and Phonegap are open source mobile development frameworks which provide access to the underlying native device API's such as the accelerometer, geolocation, and UI. Both frameworks enable application development using only Javascript, CSS and HTML. Additionally they provide development environments that work across a wide variety of devices.
Phonegap has good support for native device API's as noted here. Titanium also has similar support. There are differences between the two environments and how they expose their API's, though both provide XCode compatible projects that can be compiled and run from the XCode IDE. See the subsections for additional details and references.