We report an extensive set of results for the wetting of striped nanopatterned surfaces by simple fluids. Metropolis Monte Carlo simulations are used to investigate the validity of Cassie's law for the surface tension of heterogeneous surfaces, for the case where the surface properties vary periodically on a nanometre scale. A variety of statistical mechanical sum rules is employed to measure the interfacial free energies and a set of interfacial order parameters associated with patterned inhomogeneous fluids. We directly observe two classes of interfacial phase transitions: (i) an unbending transition at the solid-vapor interface, which must precede complete wetting in systems where low-energy regions are not completely wet; (ii) a surface crystallization-layering transition associated with a hemicylindrical region of enhanced liquid structure at the substrate-liquid boundary. For a vapor phase in contact with a striped surface we observe hemicylindrical drops pinned to the stripe boundaries, with a mechanical contact angle unrelated to that defined by Young's equation. The magnitude and variation of the mechanical contact angle with system parameters can be understood from minimal models of adsorption on patterned surfaces. (C) 2003 American Institute of Physics.