An 80 MHz pulse train of ~ 100 fs optical pulses centred at ~ 1.5 microm is propagated through a variety of high-index-contrast silicon-on-insulator waveguide structures less than 1 mm long. All-optical power limiting and negative differential transmission, based only on the intrinsic nonlinear response of the untextured waveguides near 1.5 microm, are demonstrated for average in-guide power levels of ~ 1 mW. Superlinear transmission is observed in a textured silicon waveguide for power levels less than 20 microW.