The Mobile Application Development Toolkit provides all the resources you need to start building mobile applications for Windows Mobile-based Smartphone and Pocket PC devices. Visual Studio .NET 2003 and the .NET Compact Framework enable you to build Windows Mobile-based applications quickly and efficiently, whether you want to create an entirely new mobile solution or extend a current desktop application to mobile devices.

For more information go here: http://msdn.microsoft.com/mobility/theKit/