I don't know if it's a strange question but how could I run a game made with XNA on the iPhone/iTouch ? Which steps/tools (existing ones or imaginary...) should be used ?
note: The goal is to avoid modifying existing C# code
UPDATE :
If I understand correctly, I must be able to:
1) Run my XNA code on Mono (monoxna or SilverSprite, promising?)
2) Run Mono on iPhone (MonoTouch)
For starters you would need a CLR implementation on the iPhone, which doesn't exist at the moment, but it seems someone is trying: MonoTouch.
I don't believe there is a good answer to your question. XNA doesn't target the iPhone, so the chances of being able to effectively port an XNA game without modifying the C# source code isn't likely to happen.
Instead, I'd recommend that you take a look at the various frameworks that exist to help you craft cross-platform games. Unity often comes up in these discussions, but it isn't free.
If cross-platform isn't your goal, but free iPhone development is, then I'd recommend looking at Cocos.
Edit: The MonoTouch project may be able to assist you in the future, but doesn't help you out right now. Still, it's something to keep an eye on.
The MonoTouch project may eventually help here, since it allows you to write C# targeting the iPhone (it is statically compiled to native code).
I believe XNA depends on Direct3D 9 (see http://msdn.microsoft.com/en-us/xna/aa937793.aspx), which may hint at the hurdles one might encounter in both porting to mono as well as having sufficient graphics horsepower on iPhone.
Everyone saying this is possible because there are CLR libraries for Mono, does not even think about the fact that XNA is a WHOLE DIFFERENT set of frameworks:
"The XNA Framework class library is a library of classes, interfaces, and value types that are included in XNA Game Studio."
As another responder noted, this is so far from happening it's not even funny. If you really want to write a game I'd check out Unity. It's commercial, but it looks utterly impressive and you can script your game logic in Javascript or (here's the funny part) C#!
Yes, instead of going through a ton of work to port a whole framework because you want to write a game in C#, why not just use a framework that lets you do that today?
From the horse's mouth:
MonoTouch + SilverSprite = XNA 2D games on iPhone? :)
What Bill means is that it will eventually be feasible to write a 2D XNA game, then use SilverSprite to run it on Silverlight, then use MonoTouch to run it on the iPhone
mMay I invite u to join my site www.mymonotouch.de , a new developer community,..for discussing monotouch problems
Not only is it possible but here is a video of someone doing XnaTouch on MonoTouch: http://bit.ly/13jdEV
here is the mono article about doing it http://www.mono-project.com/MonoTouch
This space is definitely heating up. There is now an XNA Touch project on codeplex that aims to bring the XNA API onto the iPhone/iPad platform:
http://xnatouch.codeplex.com/