  We generalize the notion of graded posets to what we call sign-graded
(labeled) posets. We prove that the $W$-polynomial of a sign-graded poset is
symmetric and unimodal. This extends a recent result of Reiner and Welker who
proved it for graded posets by associating a simplicial polytopal sphere to
each graded poset $P$. By proving that the $W$-polynomials of sign-graded
posets has the right sign at -1, we are able to prove the Charney-Davis
Conjecture for these spheres (whenever they are flag).
