Here are a couple of videos that start to outline a couple of different approaches to IK "spine" systems, how they work and what are the pros/cons of each. The two main things are the IK spline setup (video 1) and the ribbon spine (video 2). Again, no great detail (they're both a bit tedious to build), but enough to get you started in looking for more info.
BTW, the tutorials I mention in the videos are 1) the "divine spine" in vid 1 can be found at http://www.peachpit.com/articles/article.aspx?p=102262&seqNum=4. In the second vid I mention 2) Aaron Holly's videos (http://www.fahrenheitdigital.com/dvds/rigging/feature-animation-rigging-dvd.php) which may not even be available any more (I've never actually seen this specific topic covered by him, though I've seen other stuff and he's awesome) and 3) Jose Antonio Martin Martin's tutorial on Creative Crash (formerly HighEnd3D) (http://www.creativecrash.com/maya/tutorials/character/c/ribbon-spine-rig). I never noticed the comments before now where everyone's referring to Aaron's videos. Funny (though if he's directly ripping Aaron's techniques, he should at least credit him, IMHO. Just saying).
Maya/Rigging: intro to IK-type spine setups part 1 from zeth willie on Vimeo.
Maya/Rigging: intro to IK-type spine setups part 2 from zeth willie on Vimeo.