Inclusive pion production at high transverse momenta (p_T > 2 GeV/c) in
nucleus-nucleus (A+A) collisions at CERN SPS (sqrt(s_NN)~20 GeV) is revisited
and systematically compared to all existing proton-proton data in the same
range of center-of-mass energies. The ratio of A+A to p+p pion cross-sections
(nuclear modification factor) for 0-8% central Pb+Pb, Pb+Au and S+Au reactions
does not show a strong enhancement as a function of p_T as previously found,
but is consistent with scaling with the number of nucleon-nucleon (NN)
collisions. Neutral pion yields per NN collision in head-on Pb+Pb reactions are
suppressed, whereas peripheral yields are enhanced. These results together
indicate that some amount of ``jet quenching'' may already be present in
central heavy-ion reactions at sqrt(s_NN)~20 GeV.Comment: Updated versions of Figs. 1,2 and 4. Minor text modification