Currently the XML-RPC.NET home page says:
For the time being, XML-RPC.NET is released under the GNU LGPL licence.
Its turns out that at least one user is having difficulties with this license because of corporate policy and so I've been doing some research into using a difference license which doesn't have the copyleft properties of GPL and LGPL.
The MONO FAQ has a section on licensing which explains they are using the MIT X11 license for their class libraries. This is compatible with the BSD license. On the basis that I'd like as many people as possible to use XML-RPC.NET the MIT X11 license seems a good choice. If I was planning to make some money out of some software then I would probably go with GPL which would allow free non-commercial use but retain control over how the library was used commercially. But this has never been a goal of the XML-RPC.NET project.