Microsoft Visual Studio 2010 is an incredible development environment for Windows Phone 7, with that in mind, we need to highlight that Windows Phone Developer Tools (WPDT) CTP isn’t compatible with VS2010 retail RTM. If you install retail (RTM) VS2010 on same machine on which you already have WPDT CTP, you’ll be unable to build Windows Phone projects. Further, while Windows Phone project type will continue to appear in File/New dialog, you’ll not be able to build Windows Phone 7 apps. WPDT CTP was based on VS2010 RC, and updates made to VS between RC and RTM release causes incompatibilities with WPDT CTP. We’re working and will’ve update within a few weeks. In the meantime, to use retail VS2010 while retaining the ability to use WPDT CTP, you can “Install VS2010 to a VPC image, or Install VS2010 on another machine. Specific Notes: Installing WPDT CTP to a VPC image isn’t supported; Installer for WPDT CTP will prevent from installing on machine on which you’ve already installed VS2010 RTM; Installing retail VS2010 on a machine with WPDT CTP already installed will result in a successful install of VS2010, however, creating or opening Windows Phone Developer Tools Projects willn’t work.” note Microsoft.