... and only when that nonlinear load is the major portion (over 50%) ....
See the 2005 NEC .....
Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.
(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor.
FPN: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.
Joe Tedesco, NEC Consultant