The answer is
Apple lifted the restrictions on non-Objective C/C/C++ apps -- you just can't load code that isn't in the app bundle.
MonoTouch lets you use .NET languages -- C# is directly supported, but if you have Windows, you can make assemblies in any .NET language and use it.
There are rumors that Apple is going to support other languages directly -- I keep hearing ruby, but they are just rumors.
I think Lua is being used for game logic on a lot of apps.
EDIT (in 2018): Generally you can use any language that you can get to compile for iOS or even install language interpreters. The main thing you cannot do is load code from the Internet that wasn't in the app bundle.
People do this all of the time anyway (see React Native apps loading JavaScript from servers), but, technically, it's not allowed. The main thing that will get you attention from Apple if you make some kind of App Store that loads whole App-like things.
EDIT (in 2020): from @Pylot in comments: I know this is a long time ago, but now at least technically you can load code that isn’t embedded in the app, as you can write with JavaScript using the webview. Not staying your answer is wrong or anything, I definitely agree with you. but I was looking for something and found this post on the way. Figured if anyone sees this it might help them out.
The programming language of iOS(and Mac OS) is Objective-C and C. You have to use Xcode platform to develop iOS apps, on the next version that is now available on beta release, Xcode 4 supports also C++.
You can use "smart BASIC" programming language. It is a genuine but very advanced BASIC language with all its power and simplicity. Using its free SDK, BASIC code can be easily published as a standalone App Store application. There are many apps in App Store, written in "smart BASIC" programming language.
This might be an old thread, but I'd like to mention Appcelerator Titanium, which allows anyone versed in HTML5/JavaScript/CSS to develop iOS applications.
objective-c is the primary language used.
i believe there is a mono touch framework that can be used with c#
Adobe also is working in some tools, one is this iPhone Packager which can utilize actionscript code
With plans to slowly retire the long-used Objective-C, Apple has introduced a new programming language, called Swift, for designing apps and applications to run on Apple iOS devices and Apple Macintosh computers.
Apple says: "Swift is a new programming language for iOS and OS X apps that builds on the best of C and Objective-C, without the constraints of C compatibility. Swift adopts safe programming patterns and adds modern features to make programming easier, more flexible, and more fun. Swift’s clean slate, backed by the mature and much-loved Cocoa and Cocoa Touch frameworks, is an opportunity to reimagine how software development works."
What programming languages can one use to develop iPhone, iPod Touch and iPad (iOs) applications?
Ruby, Python, Lua, Scheme, Lisp, Smalltalk, C#, Haskell, ActionScript, JavaScript, Objective-C, C++, C. That's just the ones that pop into my head right now. I'm sure there's hundreds if not thousands of others. (E.g. there's no reason why you couldn't use any .NET language with MonoTouch, i.e. VB.NET, F#, Nemerle, Boo, Cobra, ...)
Also are there plans in the future to expand the amount of programming languages that iOs will support?
Sure. Pretty much every programming language community on this planet is currently working on getting their language to run on iOS.
Also, a lot of people are working on programming languages specifically designed for touch devices such as iPod touch, iPhone and iPad, e.g. Phil Mercurio's Thyrd language.
The SDK agreement and App store guidelines have been changed (circa Sept 2010).
You can now probably use any compiled language that will compile to the same static ARM object file format as Xcode produces and that will link to (only) the public API's within Apple's frameworks and libraries. However, you can not use a JIT compiled language unless you pre-compile all object code before submission to Apple for review.
You can use any interpreted language, as long as you embed the interpreter, and do not allow the interpreter or the app to download and run any interpretable code other than code built into the app bundle before submission to Apple for review, or source code typed-in by the user.
Objective C and C will likely still be the most optimal programming language for anything requiring high performance and the latest API support (* see update below), as those are the languages for which Apple targets its iOS frameworks and tunes its ARM processor chipsets. Apple also supports the use of Javascript/HTML5 inside a UIWebView. Those are the only languages for which Apple has announced support. Anything else you will have to find support elsewhere.
But, if you really want, there are at least a half dozen BASIC interpreters now available in the iOS App store, so even "Stone Age" programming methodology is now allowed.
Added: (*) As of late 2014, one can also develop apps using Apple's new Swift programming language. As of early 2015, submitted binaries must include 64-bit (arm64) support.
Only Objective-C is allowed by now... but since a few months ago you are allowed to write scripts that will be interpreted in your application.
So you may be able to write a LUA interpreter or a Python interpreter, then write some part of your application in this scripting language. If you want your application accepted on the App Store, these scripts have to be bundled with the application (your application cannot download it from the Internet for example)
It is also now possible to use OCaml for developing iOS applications. It is not part of the standard distribution and requires modifications provided by the Psellos company. See here for more information: http://psellos.com/ocaml/.
Source: Stackoverflow.com