The package is offered "as-is" without any warranty or commitment to support. However, informal advice and help is available via the PEAR user mailing list and XML-RPC.com.
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.
Together with Simon St.Laurent and Joe Johnston, Edd Dumbill wrote a book on XML-RPC for O'Reilly and Associates on XML-RPC. It features a rather fetching jellyfish on the cover.
Complete details of the book are available from O'Reilly's web site.
If you've benefitted from the effort he has put into writing this software, then please consider buying the book!