At this moment, I know only one way of developing .NET applications on iOS — MonoTouch platform, which is one the the versions of Mono, see:
http://en.wikipedia.org/wiki/Mono_%28software%29[
^],
http://en.wikipedia.org/wiki/MonoTouch#MonoTouch[
^]
http://xamarin.com/monotouch[
^]
In principle, if you use only standard CLR libraries +
System.Windows.Forms
and build the project on Windows targeted to .NET Framework v.3.5, this application normally works on other platforms under Mono, without re-compilation. Despite of number of minor incompatibilities, I do it in regular basis. I've done if for Linix and Max OS X, not for iOS though.
It works very well on different flavors of Linux and other systems, but it looks more or less ugly on Apple platforms, which is probably the most hostile to third-party products. Mac OS and iOS systems have different concept of UI, so your Forms application might seem not a first-class citizen.
Development of decent Apple applications are possible using the bridge product called MonobjC, see:
http://en.wikipedia.org/wiki/Monobjc[
^],
http://www.monobjc.net/[
^]. I tried it on Mac OS X only, don't know much its use on iOS, but you would certainly need to try.
—SA