I recently released a tool to convert MusicXML to ABC. You can find it at http://code.google.com/p/mxml2abc/
It works well on single and multi-part MusicXML, but does not cope too well with some advanced features. I ran it on a collection of 1700 MusicXML carols and about 1600 produced reasonable ABC. Do note that ABC does not provide all the features of MusicXML, so it is not reasonable to expect all files, or features within files to convert successfully.
There will be an improved version quite soon ;)