  The transverse single-spin asymmetry A_N observed in high energy
proton-proton collisions p^\uparrow p \to pi X has been found to increase with
the momentum fraction x_F of the pion up to the largest measured x_F \sim 0.8,
where A_N \simeq 40%. We consider the possibility that the asymmetry is due to
a non-perturbatively generated spin-flip coupling in soft rescattering on the
target proton. We demonstrate using perturbation theory that a non-vanishing
asymmetry can be generated through interference between exchanges of even and
odd charge conjugation provided both helicity flip and non-flip couplings
contribute. Pomeron and odderon exchange can thus explain the energy
independence of the asymmetry and predicts that the asymmetry should persist in
events with large rapidity gaps.
