Marvel announced the Journey Into Mystery/New Mutants crossover event at one of their Next Big Thing Liveblogs (they will have more this week)...
http://marvel.com/news/story/18103/next_big_thing_liveblogs3:05 UltimateKidNova:
We're joined by Kieron and Andy as they will be working as writers on Exiled, a crossover coming this May between Journey Into Mystery and New Mutants (two of my favorite books)!
3:05 UltimateKidNova:
It's a five-part story kicking off in the Exiled one-shot then bouncing back and forth between two issues of JiM and NM both out in May
3:08 UltimateKidNova:
"There's a history between the New Mutants and Asgard." -Kieron Gillen
3:09 UltimateKidNova:
This will draw all the way back to the Siege tie-in issue of New Mutants that Kieron wrote.
3:10 UltimateKidNova:
"This developed quite quickly and quite easily given the long history of the New Mutants in Asgard. We wanted to bring that to Earth rather than take them back to Asgard. It's a little twist on how past stories have played out. There are some really cool reveals and surprises along the way that will hopefully play against type and expectation." -Andy Lanning
3:12 UltimateKidNova:
Carmine Di Giandomenico will be doing the interiors for the story.
3:18 UltimateKidNova:
Kieron explaining the story of the Disir, the cursed former handmaidens of Bor who became a sort of "ghost story for Asgardians"
3:32 UltimateKidNova:
Mephisto will indeed be a part of Exiled. He's currently in possession of the Disir and just had a date with the New Mutants' Magma.

Exiled art by Stephanie Hans