The game is built in AS3 (yes, Flash!) using the Flare3D commercial engine. The game will be compiled using Adobe AIR Captive to native executables for Windows and Mac, so it will function just like any desktop application. The engine is powerful, supporting fullscreen hardware acceleration, dynamic lighting and shaders and we hope to release this on Steam. Currently there are quite a few games on Steam using the Adobe AIR tecnology, such as Binding of Issac.
If the multiplayer stretch goal is reached - the backend will be built on the .NET platform.