If you set logging level to INFO you get:

INFO: Missing classdef for creator 'spring'. Failed to load uk.ltd.getahead.dwr.create.SpringCreator. Cause: org/springframework/beans/factory/BeanFactory
Feb 8, 2006 1:02:23 PM uk.ltd.getahead.dwr.util.CommonsLoggingOutput info
INFO: Missing classdef for creator 'script'. Failed to load uk.ltd.getahead.dwr.create.ScriptedCreator. Cause: org/apache/bsf/BSFManager
Feb 8, 2006 1:02:23 PM uk.ltd.getahead.dwr.util.CommonsLoggingOutput info
INFO: Missing class for converter 'hibernate'. Failed to load uk.ltd.getahead.dwr.convert.HibernateBeanConverter. Cause: Failed to find either org.hibernate.Hibernate or net.sf.hibernate.Hibernate.
Feb 8, 2006 1:02:23 PM uk.ltd.getahead.dwr.util.CommonsLoggingOutput info
INFO: Missing classdef for converter 'jdom'. Failed to load uk.ltd.getahead.dwr.convert.JDOMConverter. Cause: org/jdom/Document
Feb 8, 2006 1:02:23 PM uk.ltd.getahead.dwr.util.CommonsLoggingOutput info
INFO: Can't marshall org.jdom.Document because converter 'jdom' is not available. The converter definition may be missing, or required element may be missing from the CLASSPATH
Feb 8, 2006 1:02:23 PM uk.ltd.getahead.dwr.util.CommonsLoggingOutput info
INFO: Can't marshall org.jdom.Element because converter 'jdom' is not available. The converter definition may be missing, or required element may be missing from the CLASSPATH

Doesn't seem to cause any problems, but why does it appear?