I have some rather boring dark mahogany finish bookcases and really wanted to do something to punch them up a bit. Painting the back of them makes the objects in front pop more, adds interest to an otherwise boring bookcase, and can be a fun weekend project.
See below for some fun examples, including putting fabric on the back!
Living room by Jason Bell in House Beautiful. I pretty much love this entire living room. Everything.
Hopefully this bookcase distracts from that leopard sofa.
http://www.dominomag.com/howtos/diy/2007/04/howto_paint_bookshelves DIY project from Domino Mag.