  We present numerical evidence that there are two distinct universality
classes characterizing driven interface roughening in the presence of quenched
disorder. The evidence is based on the behavior of $\lambda$, the coefficient
of the nonlinear term in the growth equation. Specifically, for three of the
models studied, $\lambda \rightarrow \infty$ at the depinning transition, while
for the two other models, $\lambda \rightarrow 0$.
