Is it a spoiler to discover which member of the Fantastic Four died?
Well, yes, I suppose. But let's face it, it's not much of a spoiler. It's just a publicity stunt.
Readers have already been "spoiled" because we've known for a while that one of the FF was going to die. Which means it's already spoiled any shock value over the character's death, whoever it may be. There was at least a 25% chance we'd be able to guess who was going to die anyway. Finding out who it is would now surely just result in a shrug of the shoulders and the usual response of "How long will the character stay dead?"
Of course, what we don't know (until we read the book) is how the character died and why. And what effect it has on the rest of the team and other characters in the Marvel universe. If a story can be spoiled just by knowing who's going to die, then IMO it's not much of a story. The death scene can occur on one page of comicbook - so what are the other 21 pages (or however many pages there are in FF #387) about?
Also, I like Joe Quesada's comment about not saying if the character will ever return from the dead and adding "but I can assure you that it's going to be very, very interesting and not what anyone expects". Which kind of implies that it will happen, but in a way no one expects.
So, I guess that means the remaining Thantastic Three aren't going to journey into the afterlife to 'rescue' their dead companion's soul? I mean, they did it with Ben Grimm a few years ago so that's not unexpected. Maybe it's all a dream? ;)