Remember, .NET Framework 3.0 = .NET Framework 2.0 + WPF + WCF + WF + WCS. So the installer first installs version 2.0 of the .NET Framework if you don't already have it installed. After that it adds the WinFX technologies on top of it.
If you download the full version (x86), you'll see that it weighs in at a hefty 45.8 MB. If you crack open the file in WinZip, you see that it is composed of multiple installers for the different components mentioned above:
Does it look like four installers, without even a real naming convention, slapped together to you too? 🙂
When you uninstall version 3.0 it leaves version 2.0 of the .NET Framework installed, because the uninstaller cannot determine if it is no longer in use. You can manually remove version 2.0 if you like, but why would you 😉
There is also an accompanying build of the Windows SDK for the .NET Framework 3.0, the Microsoft Visual Studio Extensions - Development Tools for .NET Framework 3.0® and the Visual Studio 2005 Extensions for Windows WorkFlow Foundation.
[Update 2006-06-24: The Windows SDK doesn't seem to be available for download yet (at least not in Europe), so the link might not work. The large downloads tend to replicate slowly through Microsoft's download servers.]
[Update 2006-06-26: The download for the Windows SDK is now working (1.1 GB). As an alternative you might want to check out the setup program that downloads only selected parts of the SDK when it becomes available.]