Today, Marvel put out a press release for the upcoming "X-Force" #26, where "Second Coming" climaxes in the death of one of the X-Men:

With the translucent figures standing over a dead body, we've finally got a full roster of which characters may be taking the big (albeit most likely temporary) sleep, and while we've already voiced our theories on which one it might be, a close examination of the ad gives us a pretty strong clue.
Here, take a second look:

"X-Force" #26 hits shelves on April 28, written by Christopher Yost and Craig Kyle with pencils by Mike Choi and covers by Adi Granov, John Cassaday, David Finch and Greg Horn.