The modifications available here are based on the version 1.4.4 stable of the pear XML_RPC lib. They are already sent to the lead maintainer and will hopefully find their way into the package. Until then you might want to check out this fully API compatible package.
Major enhancements are:
the implementation of the so called multicall support (explanation), which is essential for webscripts, as one call could easily result in many more requests to the serving script, if they can't be bundled.
the server script now supports simple http authentification scheme which enables the definition of user/password combinations allowed to execute the server api.
one can define and execute static class functions and server object methods from servers alias map now.
As the author didn't document the changes neatly (yet;) an example server script (in the doc folder of the package) shall be a good start to explain the usage of the server API.