Copied from www.netfx3.com:
"The .NET Framework 3.0 has officially been released! You can download the .NET Framework 3.0 components here:
Note, if you are using Windows Vista the .NET Framework 3.0 Runtime Components are installed by default.
The Readme for the released version of the .NET Framework 3.0 is available here. If you have a previous CTP installed, please be sure to review the uninstall instructions. If you have questions about installing the .NET Framework 3.0, please post your questions to the .NET Framework Setup Forum."