Death in comics is pretty much a universally impermanent concept. But a death usually lasts a good while before the character comes back, with a couple exceptions.
Well, Marvel's X-Men editor Mark Paniccia has thrown out a teaser that another character who died very recently may be back sooner rather than later.
Jamie Madrox aka Multiple Man died at the very start of Death of X, the event that preceded Inhumans vs X-men which brought us to the ResurrXion branding that has seen both X-Men and Inhumans properties taking new directions. Well, apparently we may see this resurrection taken literally with Madrox's return. Funnily enough, all the series' had writer Charles Soule involved.
Of course, we also know that the big bad of Astonishing X-Men is the Shadow King, a telepathic parasite that revels in illusions and nightmares, and previously teased artwork from Mike Deodato, the artist of the second issue, implied a 'stage play' involving X-Men characters, which could be a mental illusion or, hell, maybe someone in the Marvel Universe has actually decided to put on a play about the X-Men. Maybe it's a musical.
At any rate, this could potentially be a tease for the characters return or just a tease, period. But fans of X-Factor and Madrox may want to pick up Astonishing X-Men #2 to find out.
[UPDATE] Updated with corrections with thanks to eagle-eyed continuity buff @quizlacey