The news has already broken across the Internet, but I'll spoil it after the jump. The soon-to-be ex-hero is...
Johnny Storm, a.k.a. The Human Torch, the youngest member of the Fantastic Four. By process of elimination, it makes sense that the Torch would be snuffed out. Ben Grimm, a.k.a. the rock-covered Thing, is probably the team's most beloved member (and has prominently appeared in promotional material as a member of the New Avengers). It seems unlikely that the writers would kill off the group's only female member, Susan Storm Richards, a.k.a. the Invisible Woman (it's not "Invisible Girl" any more, is it?). Reed "Mr. Fantastic" Richards could have been the most poignant death. The current conception of the character focuses less on Richards' stretching powers than his super-genius intellect and the burden he bears as a man who could rid the world of its problems. But it's hard to imagine the team even existing without him.
(Coincidentally enough, Chris Evans played the Human Torch in the recent two Fantastic Four feature films, and is now playing Captain America in this summer's new superhero flick.)
Deaths are never permanent in comic books, especially with iconic characters, and The Fantastic Four was the first superhero comic Marvel ever published. After the post-Torch fallout, it'll be interesting to see how long he stays dead, and whether they come up with a clever means of bringing him back. My pet theory: that the Fantastic Four revives the original Human Torch, an android who first appeared in 1939, but is currently dead in Marvel continuity.
At any rate, I'll bet that bereaved New Yorkers will be raising their cigarette lighters outside the Baxter Building this week...