[en] Previous total-energy calculations for bulk Ga_0.5In_0.5P alloys have demonstrated that the lowest-energy configuration at T=0 corresponds to phase separation into GaP+InP, followed by the ordered GaInP₂ chalcopyrite phase as the next lowest state; the (111)-ordered CuPt-like superstructure is predicted to lie at a much higher energy. Yet, vapor-phase crystal growth has shown CuPt-like long-range ordering in relatively thick Ga_0.5In_0.5P films grown on a lattice-matched (001) GaAs substrate. We present here first-principles local-density total-energy calculations for Ga_0.5In_0.5P/GaAs(001) in various two-dimensional structures, each having a free surface. For one-monolayer coverage, we find electronically driven surface reconstructions, consisting not only of the previously known cation dimerization, but also of buckling and tilting of the surface dimers. These considerably stabilize the CuPt-like surface topology over all other forms of surface order, including phase separation. Furthermore, a Ga/In layer covered by three monolayers still exhibits a significant energy preference (relative to kT_g, where T_g congruent 900 K is the growth temperature) for the CuPt structure. If complete atomic mobility were to exist irrespective of how deeply buried the atoms are, we would then expect that the surface-stable CuPt ordering would exist in the near-surface regions, whereas deeper layers would revert to the bulk-stable structures. Since, however, surface atomic mobilities are far larger than bulk mobilities, it is possible that surface-stabilized structures will be frozen in and consequently ordering will propagate into macroscopic film dimensions. In light of our results, we describe several possible ways that surface effects could lead to long-range CuPt-like ordering