You need to use some language or framework to make it cross-platform, and since the majority of people run Windows and I run Linux but there has never been a native Linux army creation application then cross-platform compatibility is an important requirement. Java is one funky language, .Net is a framework for any number of funky languages.
As for the iPhone, it's possible to run Mono on it (although Apple may lock it out because they're like that with their gadgets). Basically I think anything that has Java probably has Mono, or will do soon. TBH I don't place much value on the iPhone though, it's just another flashy Mac gadget. Getting a decent army creation application to run on Windows, Mac and Linux will be better than just about anything that has come before.
I will have to ask, though, what is "GUI inter-phase experience"? Do you mean interface, and how much actual coding do you know/have you done? You could put together an interface without coding, but without the coding knowledge then you won't get the bindings to the underlying library.