So i was with this guy, furiously writing notes, and the app looks like a straight-on C#.NET desktop app that stores entered data on the local drive and then transmits it later to a server on the 'net.

Piece of cake.  No. easier than that.  A piece of a cupcake.

The Catch

Then my pen hits the notepad, and I rub my eyes and get ready to shake his hand and he says, "Of course this is going to work on Windows and Mac, right?"

I said "No.  Windows only.  You said 'a windows app' in your email, and your existing app is a DOS app, so why would I even go there in my head?"

"Oh, no.  It's got to work on a Mac, AND it can't use emulation mode either."

What's the solution?

Ok, Mighty Developers, tell me: what would YOU use for this?

Keep in mind that the desktop app must run WITHOUT internet connectivity, and we can't install a massive, expensive app like Paradox. 

Also, I do NOT want to buy a Mac and do parallel development.