What's the minimum that I need to have installed on my Build Server (in addition to the standard .Net 3.5 stuff) to allow it to compile Silverlight 2.0 applications?
I have a Silverlight application that seems to be building correctly, but is not playing nicely with a related Web Application project - see this related question that I asked earlier: Silverlight xap file not being copied to ClientBin on Build Server
Your build server should be pretty much an exact replication of your development machine as far as plug-ins, assemblies, run-times, SDK's etc.
You build server will be building your app via MSBuild.
In your particular case I suspect that you need to install the Silverlight Toolkit. This will give you the SL development runtime and the appropriate project build requisites for your XAP files to build and be deployed properly.
HTH, Mark
AFAIK you need Microsoft Silverlight 2 SDK to install on the build server.
HTH