Check this plugin out:
http://nubyonrails.com/articles/2005/12/27/dump-or-slurp-yaml- reference-data
Careful when you use it; it overwrites your text fixtures.
You might want to just use part of the plugin's code to only do the
to yaml bit instead. Have a look here:
http://topfunky.net/svn/plugins/ar_fixtures/lib/ar_fixtures.rb
-christos