Microsoft is currently only redistributing a single installer package for Silverlight. While the complete set of Visual Studio tools is great for my workstation, my goal is not to install Visual Studio on the build server I'm in the process of configuring. While Microsoft doesn't explicitly distribute the SDK installer in it's own package, it's easy to extract from the Silverlight Tools installer.

After downloading the installer, open up a command line and navigate to the directory where the file exists. Simply execute the Silverlight_Tools.exe file with the /? flag, and take note of the dialog box which pops up:

The "To Directory" row will show you the directory the files are being extracted into. Open up explorer and navigate to that directory. After a few moments, a "Silverlight Tools Installation Wizard" dialog will open, providing usage instrutions for the command line arguments valid against the Silverlight_Tools.exe. Do not close the window, but move it out of your way.

Grab the silverlight_sdk.cab and the silverlight_sdk.msi files and copy them out of this directory. Once that is done, you are free to close that dialog from the previous paragraph. It will automatically delete the temp directory it created.

Now, just copy those two files to whatever system you want the SDK, but not the Visual Studio tools, and double click on the MSI. It will install the SDK files, and you're MSBuild scripts will now be able to build Silverlight projects. Hopefully Microsoft will soon release an SDK-only installer for just this situation. But then, hopefully Microsoft will also make MSTest installable without installing at least 1.7 GiB of Visual Studio.