Disclaimer: There's a 50/50 chance of another poly person disagreeing with everything and anything I say in this article. I love to hear from other poly people—especially when I'm wrong.
Yes, jealousy occurs in poly relationships. We're no more excluded from it than a monogamous couple. The form it takes depends on the situation and people involved. Examples I've personally witnessed range from the traditional, "I'm afraid you'll find someone better and leave me," to "I'm not jealous. I just don't like that person and don't want you to date them." The most common form of jealousy revolves around Partner A being so wrapped up in New Relationship Energy (aka "shiny, new person" or "honeymoon" phase) that Partner B feels neglected.
This article appears in Aug 26 – Sep 1, 2009.