This question has been answered quite clearly for MonoTouch here: http://stackoverflow.com/questions/2604033/is-monotouch-now-banned-on-the-iphone But what about Appcelerator Titanium? The new TOS from Apple and their iPhone 4 OS: 3.3.1 — Applications may only use Documented APIs in the manner prescribed by Apple and must not use or call any private APIs. Applications must be originally written in Objective-C, C, C++, or JavaScript as executed by the iPhone OS WebKit engine, and only code written in C, C++, and Objective-C may compile and directly link against the Documented APIs (e.g., Applications that link to Documented APIs through an intermediary translation or compatibility layer or tool are prohibited). Titanium uses JavaScript but is not executed be the iPhone OS WebKit engine directly. In their Developer blog, Jeff Haynie says Titanium is on the clear, but I don't know if they are in denial. It’s our belief that we are fully in compliance with iPhone OS 4.0 ToS as we interpret them. I haven't found any official word by Apple, only opinions. And I'm quite confussed. I'm not writing another line of code for my App until... you know.