  We study the number of visits to balls B_r(x), up to time t/mu(B_r(x)), for a
class of non-uniformly hyperbolic dynamical systems, where mu is the SRB
measure. Outside a set of `bad' centers x, we prove that this number is
approximately Poissonnian with a controlled error term. In particular, when
r-->0, we get convergence to the Poisson law for a set of centers of mu-measure
one. Our theorem applies for instance to the H\'enon attractor and, more
generally, to systems modelled by a Young tower whose return-time function has
a exponential tail and with one-dimensional unstable manifolds. Along the way,
we prove an abstract Poisson approximation result of independent interest.
