Bobby Woolf mentioned in a blog today that he wrote an article for the WebSphere Technical Journal about ... oh hell, I can't explain it briefly - just check out the article.
Seriously though, if you'd like to gain a better understanding about what JNDI is for and the service locator pattern (and a nasty potential problem with it in J2EE 1.3), check out the article.
PS - on a related side note, Martin Fowler has a typically insightful article talking about the Dependency Injection (a.k.a. Inversion of Control, or IoC) pattern and how it compares to the Service Locator pattern that Bobby talks about. Thanks to Ted Neward for telling me what IoC stood for since Google couldn't.
Bobby Woolf focuses on a problem with caching in the service locator pattern (J2EE 1.3 and later)