For a fairly general class of two-dimensional tiling substitutions, we prove
that if the length expansion $\beta$ is a Pisot number, then the tilings
defined by the substitution must be locally finite. We also give a simple
example of a two-dimensional substitution on rectangular tiles, with a
non-Pisot length expansion $\beta$, such that no tiling admitted by the
substitution is locally finite. The proofs of both results are effectively
one-dimensional and involve the idea of a certain type of generalized
$\beta$-transformation.