Yes, it uses Mono with MonoMac (http://www.mono-project.com/MonoMac). We built a platform-independent core library and application controller, then simple Windows.Forms and Cocoa views to support Windows and Mac OS.
Our goal is to build a large collection of small software products, including a mix of desktop and web apps, that help people form sophisticated relationships with technology. We also want to build "hardware" to support people in making healthy changes to technology habits.
Our first product, Awareness, is a C# app for Mac and Windows, already distributed through the App Store.
I'm actually looking to bring another programmer onboard, so send us your resume!