It can't be Jack or Kate in the box, unless we're dealing with time travel or alternate intersecting (not parallel) universes. But let's assume for the sake of this post we're not.
The crumpled newspaper clipping in Jack's hand may say "J... ....tham" - according to a screen shot all over the Web. Jeremy Bentham was one of the founders of the Utilitarian philosophy in the 19th century - the path to goodness consists of finding or doing the greatest good for the greatest number of people. Bentham developed this philosophy with James Mill, whose son, John Stuart Mill, refined it (JS Mill argued, correctly I think, that there are qualitative differences between goods - saving someone's life is worth much more than giving someone $100 as charity). This philosophy is certainly not irrelevant to the castaways, and the whole story of Lost.... Bentham is also known for devising the Panopticon prison - a design which allows the prison guard to observe all prisoners, who are unaware that they're being observed - a state also not exactly alien to Lost.... In any case: if the name in the article is indeed Jeremy Bentham, this would be consistent with major characters in Lost having the complete or partial names of great philosophers - John Locke, Hume, Rousseau.
But this of course wouldn't tell us who the person in the coffin is - that corpse could either be of a new character, to be introduced after the last scene we saw on the island, or an alias being used by a character we know.
Intriguing possibilities, both. But let's look at some other alternatives:
Michael: This is supported by the funeral home being in an African-American neighborhood. That's about it for evidence.
Locke: The "J" in the newspaper clipping could as easily be the first letter of John as Jeremy.
Juliet: Same logic as above. But the crumpled article indicates that a man died - which would rule out Juliet, as well as Claire, Rose, and Sun ....
Here's another factor: Kate seems to care less about this person than does Jack ... who from the island would Kate dislike and Jack not? Almost impossible to say at this point, because of the time that has passed since we last saw the two on the island.
So, I'd say we need to know much more to constructively guess who's in the box ... which of course is what the producers intend .... Anything I've overlooked?
*Note added 27 May 2007: Some people think they see an "a" as the second letter of the last name. If that's indeed what it is, maybe it's a typo...:)
**Note added 29 May 2008: And the body in the coffin was revealed in the Season 4 finale ... here's my review and analysis ...
Lost New Questions1. How Far in the Future? ... 3. Who's Waiting for Kate? ... 4. Who Is Naomi's Boss? ... 5. Is Mikhail Immortal? ... 6. What Constitutes Reliable Evidence? ... 7. Are the Flashforwards Desmond's Flashes?
Lost Season 3 Finale ... Flashforwards
Lost: Keys to What's Really Going On
And see also Lost's Back Full Paradoxical Blast: Season 4 Episode 1
Utilitarianism and Other Essays John Stuart Mill and Jeremy Bentham
The Plot to Save Socrates
"a Da Vinci-esque thriller" - New York Daily News
"Sierra Waters is sexy as hell" - curled up with a good book