Spin-transfer switching with short current pulses has been studied in spin-valve nanopillars with perpendicularly magnetized free and reference layers. Magnetization switching with current pulses as short as 300 ps is demonstrated. The pulse amplitude needed to reverse the magnetization is shown to be inversely proportional to the pulse duration, consistent with a macrospin spin-transfer model. However, the pulse amplitude duration switching boundary depends on the applied field much more strongly than predicted by the zero temperature macrospin model. The results also demonstrate that there is an optimal pulse length that minimizes the energy required to reverse the magnetization.