Taken from ask yahoo:
Gwarv's Briefcase page quotes Samuel Jackson in a 1995 Playboy interview, "When I looked inside, between scenes, I saw two lights and some batteries. What I would have wanted to see are the next ten films I'm going to do and hope that they're all as good as Pulp Fiction."
What's the final verdict? It doesn't matter. Alfred Hitchcock coined the term "McGuffin" to refer to a diversionary plot device -- the bone thrown by the burglar to distract the watchdog of the mind. Like the statue in The Maltese Falcon, or the stolen money in Psycho, the briefcase in Pulp Fiction serves to set the characters in motion and throw some added intrigue into the mix. It's a great trick and, obviously, it worked.
and a breakdown of the soul theroy at snopes:
http://www.snopes.com/movies/films/pulp.htm
I, for one, tend to agree that it hardly matters. The journey was what was important, not the destination.