Visual Studio 2010 does not support mobile application development for versions of Windows Phone prior to Windows Phone OS 7.0.
For Visual Studio 2008 mobile developers, Microsoft released the Windows Mobile 6.5 Developer Tool Kit, which works with the Windows Mobile 6 SDK.
Because Visual Studio 2010 does not support mobile application development for Windows Phone prior to Windows Phone OS 7.0, you cannot use the following features: .NET Compact Framework projects, Visual C++ device projects, smart device CAB projects, Device Emulator and Device Emulator Manager, testing tools for device projects, and Device Security Manager.
Mobile application development is still supported in earlier versions of Visual Studio, such as Visual Studio 2008. For more information about smart device projects, see Windows Mobile Developer Center and Smart Device Development in Visual Studio 2008.