  We compute the time evolution of elliptic flow in non-central relativistic
heavy-ion collisions, using a (2+1)-dimensional code with longitudinal
boost-invariance to simulate viscous fluid dynamics in the causal
Israel-Stewart formulation. We show that even ``minimal'' shear viscosity
eta/s=hbar/(4pi) leads to a large reduction of elliptic flow compared to ideal
fluid dynamics, raising questions about the interpretation of recent
experimental data from the Relativistic Heavy Ion Collider.
