Message-passing is a fundamental part of Erlang. Alan Kay has argued that message-passing is a concept more important than objects in his view of object-oriented programming (and he "invented" the term!).
Can Erlang be considered an object-oriented programming language (à la Smalltalk)?
Answer
Joe Armstrong has gone on record saying that he thinks that Erlang is "possibly the only object-oriented language" (the context adds "OO in the Alan Kay meaning of the word"). Johnson, in the same interview, points out the same things that Sean Copenhaver says in his answer: in the small, Erlang's a purely functional language; in the large, with processes, it looks exactly like Kay-style object orientation.