I don't know about a full YAML implementation, but JSON (a subset) is getting pretty popular and there are even some Delphi libraries listed on the JSON site. Delphi 2009 is even using (a slightly modified version of?) JSON in DataSnap.
http://sourceforge.net/projects/lkjson
http://www.progdigy.com/?page_id=6
http://sourceforge.net/projects/is-webstart/