If your customers are asking you about a “Mac” version of your existing Windows application, you may want to look at the new Embarcadero FireMonkey application development platform. FireMonkey makes it very easy to create great looking, native, compiled applications for Mac as well as for Windows. FireMonkey is a great way for building clients application with advanced GUI features like programmable effects and animations to your existing web services implemented with Microsoft Visual Studio 2010.
RAD Studio XE2 comes with multiple compilers, including Windows 32/64-bit compilers and native OS X compiler. Basically you just need to select active target platform and recompile your project and you can build applications that natively run on Windows and OS X.
That’s truly unique capability. Being able to target Windows and OS X from the same Delphi or C++Builder code is something that no other IDE on the market can currently provide.
I have put together a tutorial and a demo that features a simple ASP.NET 4.0 C# web service and a Mac OS X client application for it built with Delphi XE2 and FireMonkey. I have also recorded the whole process of building the ASP.NET web service and Mac client for it using Camtasia. The recording is just one 25-minute long mp4 file that you can download from the Code Central.