The License of the file in wordpress has been questioned in the recent WordPress GPL, licensing and copyright related issues covered partially on my blog.
According to Incutio Ltd., the company behind the XML-RPC Library, it was previously licensed under the non-GPL-compatible original Artistic License.
The now available alternatively licensed code makes it possible to end some of the licensing problems with the XML-RPC code in WordPress.
Update: New available information made me shorten this article. There is a chance that using the code was ok under Modified BSD which would help a lot for the past releases.
The related ticket is #14703.