  We establish global well-posedness and scattering for solutions to the
mass-critical nonlinear Schr\"odinger equation $iu_t + \Delta u = \pm |u|^2 u$
for large spherically symmetric L^2_x(\R^2) initial data; in the focusing case
we require, of course, that the mass is strictly less than that of the ground
state. As a consequence, we deduce that in the focusing case, any spherically
symmetric blowup solution must concentrate at least the mass of the ground
state at the blowup time.
  We also establish some partial results towards the analogous claims in other
dimensions and without the assumption of spherical symmetry.
