  The problem of calculating the Casimir force on two conducting planes by
means of the stress tensor is examined. The evaluation of this quantity is
carried out using an explicit regularization procedure which has its origin in
the underlying (2+1) dimensional Poincar\'{e} invariance of the system. The
force between the planes is found to depend on the ratio of two independent
cutoff parameters, thereby rendering any prediction for the Casimir effect an
explicit function of the particular calculational scheme employed. Similar
results are shown to obtain in the case of the conducting sphere.
