XML-RPC for PHP is offered "as-is" without any warranty or commitment to support. However, informal advice and help is available via the XML-RPC for PHP website and mailing list and from XML-RPC.com.
The PHP XML-RPC interest mailing list is run by the author. More details can be found here.
For more general XML-RPC questions, there is a Yahoo! Groups XML-RPC mailing list.
The XML-RPC.com discussion group is a useful place to get help with using XML-RPC. This group is also gatewayed into the Yahoo! Groups mailing list.