You may be asking the obvious question, i.e., "How? How will Sara return when she doesn't have a freaking head?!" Well, Olmstead makes it clear that isn't Sara's head after all, and she's been alive, although not necessarily well, this whole time.

For you who don't know, Sarah Wayne Callies couldn't return to shoot Sara's death scene last season, so producers used a Polaroid photo of her from a wardrobe test. How did the Company turn that into a fake noggin? Watch the show!
http://www.eonline.com/gossip/kristin/detail/index.jsp?uuid=3fd3e7cf-0758-484e-beca-5d5b7e09cae0