This tutorial includes a part for creating an entry with data that has a content type of text/html:
entry = new SyndEntryImpl();
entry.setTitle("ROME v3.0");
entry.setLink("http://wiki.java.net/bin/view/Javawsxml/Rome03");
entry.setPublishedDate(DATE_PARSER.parse("2004-07-27"));
description = new SyndContentImpl();
description.setType("text/html");
description.setValue("<p>More Bug fixes, mor API changes, some new features and some Unit testing</p>"+
                     "<p>For details check the <a href=\"http://wiki.java.net/bin/view/Javawsxml/RomeChangesLog#RomeV03\">Changes Log</a></p>");
entry.setDescription(description);
entries.add(entry);
I think that should be transformed into a content:encoded element in the generated feed, perhaps depending on the feed type.
EDIT: this was my first answer, but it has nothing to do with ROME:
Don't know if this is what you're looking for, but the RSS Best Practices Profile describes what should be in the element:
  The content:encoded element defines
  the full content of an item
  (optional). This element has a more
  precise purpose than the description
  element, which can be the full
  content, a summary or some other form
  of excerpt at the publisher's
  discretion.
  
  The content must be suitable for
  presentation as HTML and be encoded as
  character data in the same manner as
  the description element.