I only heard about this in this week's X-Position with Kyle & Yost.  It doesn't make sense for Warren Ellis to use her in Astonishing, instead of having her as a central plot-point & key figure in Necrosha.   
 
I'm not up-to-date with Astonishing X-Men, so I don't know how she's back or if it's permanent.  No spoilers please.  I'll get to it in my own time :)

She's back in Necrosha!?!?! 

@John Valentine:Seems she wont be. 
 
I was hoping that Wallflower would play a role in "Necrosha," but it looks like Warren Ellis is already using her in his current "Astonishing X-Men" arc. I am curious though if you guys had planned to have Selene bring Wallflower back during "Necrosha" since she was an important character during your "New X-Men" run...

Yost: I think her presence will be felt in "Necrosha."

Kyle: It already is. I think it's also fair to say that she was definitely a character we thought we might've used more, but when someone like Warren Ellis decides they want to use a character - which they have every right to do, and obviously when it's Warren Ellis, you just get the hell out of their way and let them do their genius - you know, that's when you adjust your storyline. We're just thrilled that someone else decided to pick her up and dust her off - literally - and if it's going to be anyone, Warren Ellis is a hell of a candidate.

Yost: I heard it was part of his Marvel contract. He got to use Wallflower or he was out.

Kyle: [Laughs] We love Warren Ellis. He was one of the few people, when we were being reamed on the boards for blowing up that stupid bus, who came to our defense. Never met the guy, never talked to the guy, and he was out there telling people to shut up in the most creative way I'd ever seen. And, beyond just being a fan of his work, he's like my personal hero because he stood up when he didn't have to. I mean, he didn't know us from anybody and he stuck up for us, so I am happy to sing his praises whenever given the opportunity. Big fans over here from C&C...

Yost: Huge!