from spoilerfix.com
Executive producers Damon Lindelof and Carlton Cuse reveal: It's always possible [Shannon] could pop up in someone's backstory. But she is definitely dead. When a character dies on the island, they stay dead. In the same moment that we decided Shannon would die, [we also decided that] Ana-Lucia would be responsible for that death. It would be the first time one castaway was responsible for killing another, and it would give so much inherent conflict and trauma [heading to] the merge. We're walking a very tenuous tightrope with her, but we feel that over the course of the next two episodes the audience will hopefully get a better understanding for that character and what she's been through. [Shannon] was shot. Walt's disappearance and, ultimately, Michael's efforts to reunite with him were part of a grander plan.
so yeah shannon is dead and ana lucia killed her. you can read the rest of the spoilers there, didn't wanna post all for those that haven't gotten to season 2 yet.