  One of the most well-known classical results for site percolation on the
square lattice is the equation p_c + p_c^* = 1. In words, this equation means
that for all values different from p_c of the parameter p the following holds:
Either a.s. there is an infinite open cluster or a.s. there is an infinite
closed `star' cluster. This result is closely related to the percolation
transition being sharp: Below p_c the size of the open cluster of a given
vertex is not only (a.s.) finite, but has a distrubtion with an exponential
tail. The analog of this result has been proved by Higuchi in 1993 for
two-dimensional Ising percolation, with fixed inverse temparature beta <beta_c,
and as parameter the external field h.
  Using sharp-threshold results (approximate zero-one laws) and a modification
of an RSW-like result by Bollobas and Riordan, we show that these results hold
for a large class of percolation models where the vertex values can be `nicely'
represented (in a sense which will be defined precisely) by i.i.d. random
variables. We point out that the ordinary percolation model belongs obviously
to this class, and we show that also the above mentionedIsing model belongs to
it.
  We hope that our results improve insight in the Ising percolation model, and
will help to show that many other (not yet analyzed) weakly dependent
percolation models also belong to the abovementioned class.
