There was a statement by Square, I believe, that said they were planning to make it possible to resurrect her, but the plan was scrapped due to time constraint, as someone said up here.

Anyway, it may have been... I cant remember the word. It's a theory that when someone spends enoygh time in a place, an essence of them remains even after they're gone. Or maybe her spirit was just drawn there, because of all the good memories she had there. That was her favorite place, and that was where she met Cloud.