Michael Netzer has a plea on his site to DC to not kill J'onn J'onnz, which you can visit by clicking on the picture below, but I really would be amazed if DC actually killed him off. I know Rich Johnston reported that J'onn is on the "People who we could kill during Infinite Crisis" list, which makes perfect sense, as all these lists really are are "Here are characters that you could kill without severely affecting our comic book line," and J'onn is certainly not a character whose death would be THAT big of a deal. I mean, let's be frank, the guy was out of comics period for about a decade in the late 60s until the late 70s, so DC knows they CAN live without the character.

That said, I doubt they'd actually kill the guy off, because, first off, he's too minor of a character for his death to have that much of an impact, but at the same time, he's both

A. a good character,

B. a classic character (founding member of the Justice League), and (and I think this is most important of it all),

C. there's no one to replace him. You could kill Hal Jordan and have a replacement. You could kill Aquaman and have a replacement. You could obviously kill a Flash and have a replacement.

You kill Martian Manhunter - you got nuthin'.

Now, if only he could go back to his original appearance...