Last week I get the information directly from Microsoft, that at build machines SDKs must be installed to guarantee that MSBuild works well in some circumstances.
We have the problem, that a self-containing build should include all necessary build artifacts – particularly Silverlight stuff. I tried to encapsulate the special steps in own targets and a build was finally possible (see my former posts). But at local machines without installed Silverlight stuff it gives problems for problems. Because reason of time finally I decide to go the Microsoft way – they break my volition 
Only one thing I left in my DevTree: the WebApplication target. This stuff is not included in the Silverlight SDK – no idea where it comes from. To avoid the build breaks on our continuous integration server I reference this target in my DevTree self.
These lines:
<ImportGroup Condition=" '$(Configuration)' == 'Debug' ">
<Import Project="$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v10.0\WebApplications\Microsoft.WebApplication.targets" />
</ImportGroup>
<ImportGroup Condition=" '$(Configuration)' != 'Debug' ">
<!-- leider nicht von Visual Studio akzeptiert -->
<Import Project="..\..\..\tools\WebApplications\Microsoft.WebApplication.targets" />
<!-- -->
</ImportGroup>