  This letter discusses the orientifold projection of the quantum corrections
to type IIA strings compactified on rigid Calabi-Yau threefolds. It is shown
that N=2 membrane instanton effects give a holomorphic contribution to the
superpotential, while the perturbative corrections enter into the Kahler
potential. At the level of the scalar potential the corrections to the Kahler
potential give rise to a positive energy contribution similar to adding
anti-D3-branes in the KKLT scenario. This provides a natural mechanism to lift
an AdS vacuum to a meta-stable dS vacuum.
