I suspect that C#/Mono has become the de-facto second development language/environment for iOS thanks to Unity3D (an irony that doesn't escape me). It's certainly what all the iOS games programmers I know personally use (and I know a lot).
Wikipedia have Unity down as having over 1 million developers developing for iOS, Android, Windows, Blackberry 10, OS X, Linux, web browsers, Flash, PlayStation 3, Xbox 360, Windows Phone 8, and Wii U.[2]
Unity themselves claim[3]:
* 2M REGISTERED DEVELOPERS
* 400K MONTHLY ACTIVE DEVELOPERS
* 225M TOTAL WEB PLAYER INSTALLS
* 6.6M EDITOR SESSIONS JUN