Mac OS X, Pocket PC and Windows Development

[lists]

I have an application with a unique user interface that I wish to  
develop for essentially all operating systems, but primarily for  
Pocket PC, Mac OS X and Windows.  I have deliberated over various  
options and now I think I have less of an idea of what development  
platform to use now than when I started.

I would love to stick with Cocoa and use GnuSTEP to fill in for  
compatibility for the other platforms, but I don't know if this is  
viable.  Java and Mono/.NET have also crossed my path but I'm really  
just not sure and hoped to get some solid advice from folks involved  
in such arenas.

Thanks much for any help!

-George