Libel in Editorials

A question was brought up in a writing class I'm taking about whether or not opinion or editorial pieces that are published are libel for their comments considering it is an opinion piece.  My answer to this question is that yes, the author is libel for all of his or her statements.  There is a definitive line one cannot cross without proper facts to back up an accusation.  For example, one is allowed to make remarks upon a persons character without "factual" data to back it up, other than that they met the person, or have known the person and they are giving their opinion about the person as a person.  However when the author begins to make claims about another persons actions, the author then becomes libel for the statements he or she is making.  One can state that, "I didn't like him.", but cannot state, "I didn't like him because he solicits prostitution." unless the speaker/writer has hard-evidence to prove he actually solicits prostitution.  The fact that it is an opinion piece does not mean the rules of libel have changed or bent, it simply denotes to the reader that if they are to read the article they are to expect the authors opinions within the story unlike a typical straight hard-hitting, factual news-story.