I'm currently working on the next release of XML-RPC.NET. These are the main features of the release. I'll soon be releasing a beta version.
Support for <nil/>
The <nil/> XML-RPC extension will be fully supported. Null can be used in method parameters and method return values where the relevant types are either reference types or nullable types, for example string and int?. <nil/> is also supported in XML-RPC struct member values.
Silverlight and Windows Phone
The distribution will supply assemblies for Silverlight and Windows Phone.
Support for .NET Client Profile
There will be a separate new server assembly and the client assembly will work with the .NET Client Profile.