  A theorem of J. Silverman states that a forward orbit of a rational map
$\phi(z)$ on $\mathbb P^1(K)$ contains finitely many $S$-integers in the number
field $K$ when $(\phi\circ\phi)(z)$ is not a polynomial. We state an analogous
conjecture for the backward orbits using a general $S$-integrality notion based
on the Galois conjugates of points. This conjecture is proven for the map
$\phi(z)=z^d$, and consequently Chebyshev polynomials, by uniformly bounding
the number of Galois orbits for $z^n-\beta$ when $\beta\not =0$ is a non-root
of unity. In general, our conjecture is true provided that the number of Galois
orbits for $\phi^n(z)-\beta$ is bounded independently of $n$.
