In our paper [GSV], we discussed Poisson properties of
cluster algebras of geometric type for the case of a nondegenerate
matrix of transition exponents. In this paper, we consider the
case of a general matrix of transition exponents. Our leading idea
is that a relevant geometric object in this case is a certain
closed 2-form compatible with the cluster algebra structure. The
main example is provided by Penner coordinates on the decorated
Teichmüller space, in which case the above form coincides with
the classical Weil-Petersson symplectic form.