Unless a completely rebuilt client is released it won't work with C# Express. You see, C# Express builds .NET 2.0 Beta binaries and the Terrarium is built on .NET v1.1 which is an actual released product. While I hear that work was done to make certain things just work across the versions, there are some new metadata/IL features of .NET 2.0 binaries that we simply can't consume within the Terrarium. Further, .NET 2.0 binaries could never run on a .NET 1.1 machine.
In short, a separate client would have to be released and a number of work items would have to be completed to make this happen. | 
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