  Let K be a number field and let S be a finite set of places of K which
contains all the Archimedean places. For any f(z) in K(z) of degree d at least
2 which is not a d-th power in \bar{K}(z), Siegel's theorem implies that the
image set f(K) contains only finitely many S-units. We conjecture that the
number of such S-units is bounded by a function of |S| and d (independently of
K and f). We prove this conjecture for several classes of rational functions,
and show that the full conjecture follows from the Bombieri--Lang conjecture.
