I literally couldn't sleep for hours after that finale, trying to work out who was in the coffin! All I know is: it's someone who he doesn't consider a friend (from what he said to the undertaker), I'm pretty sure Kate said, "why would I want to go to HIS funeral", which means its a guy, and it would have to be someone that would make jack contemplate suicide on hearing about their death. The obvious choice would be locke or sawyer, however, there are 3 more series, each having 16 episodes, so it may be someone like jin (or even bernard!) who hasnt really come into the program...yet!