"With Mono" says Jan, "we have a low-cost server environment where everything but the UI for the two clients is the same C# code. Then we can use Visual Studio to create a DLL for the Windows client; and Cocoa, Objective C, and Interface Builder for the Mac client. We think we're the first company to create a cross-platform product with this approach."
They talk about Dumbarton, a framework that they built to create applications that mix Cocoa and Mono.
Thanks to Zac for the link.