I wrote a vb.net program a few months ago that leverages CookComputing.com's XML-RPC library.  I just upgraded to using Framework 2.0.  The upgrade wizard did an ok job with the migrate.  It did find a few items but it wasn't too bad.  My biggest offending issue was some background threads that were not updating form elements via invoking on the main thread.  I knew about this issue but I was just lazy...and this was utility for me.  However, it forced me to make it threadsafe...which I think is a good thing. 

After fixing up my code, it wouldn't run when calling a REST API.  I used an interfaced approach with the API call.  When I called to generate the proxy, I got very non-descript error message.  Did some quick research and found that there is known bug with the framework 2.0 and the library as to how it uses reflection not marked as Public.

From CookComputing.com:

In May I reported to the MSN Product Feedback Center a problem with .NET 2.0 beta 2 which broke existing XML-RPC.NET applications. The bug report was later marked as fixed, in particular "This bug will be fixed in RTM milestone.". Unfortunately the problem still exists in the release version of .NET 2.0.

Therefore any applications which create a XML-RPC.NET proxy based on an interface which is not declared as public will fail on the call to XmlRpcProxyGen.Create.