  We consider single-inclusive forward pion production in high-energy
proton-proton collisions at RHIC energies. A good baseline description of the
transverse momentum distributions at high rapidity is obtained within Mueller's
dipole formalism with an anomalous dimension incorporating an ``extended
geometric scaling'' window between the saturation and DGLAP regimes. We then
formulate pion production for transversely polarized projectiles within the
same approach. We assume that an azimuthal, spin-dependent asymmetry arises
from the so-called Sivers effect and investigate the single transverse-spin
asymmetry A_N at 200 GeV and 500 GeV center-of-mass energy. A simple
parameterization of the Sivers functions from the literature compares
reasonably well with the high-energy STAR data if the overall normalization is
scaled up by at least a factor of two. The STAR data might therefore indicate
that the Sivers effect is significantly stronger than thought so far. We also
analyze higher-twist contributions to A_N and find that they largely cancel.
