I rekon that it was Ben in the coffin, and when Jack finds out that he is dead, he realises that Ben was his last hope of getting back to the island (which he clearly wants to go back to for some reason) - which is why he tries to commit suicide.
A couple of other things that have been picked out are that the name of the funeral directors: "Hoffs/Drawlar" is an anagram for "flash forward"
The mobile that Jack uses in the flash-forwards was a Motorola KRZR - which was only released in October 2006. They crashed onto the island in 2004 - which is an indication of how far into the future the flash-forward is.
The newspaper clipping that Jack reads:
http://www.lostpedia.com/images/thumb/d/d4/Newspaperclipping2.jpg/200px-Newspaperclipping2.jpgsays:
Los Angeles
Man found dead in
downtown loft
"The body of Jo...[unreadable]...[a]ntham of
New York was discovered shortly after 4
a.m. in the...[unreadable]...of Grand
Avenue. Ted...[unreadable]...man at The
Tower...[unreadable]...heard loud
noises...[unreadable]...antham's loft.
[unreadable]... [sa]fety, he
[unreadable]...discovered the
[unreadable]...a beam in the
[unreadable]...[ac]cordin[g]...[unreadable]..."
Interesting...