  Vacuum polarization integrals involve the vector spectral functions which can
be experimentally determined from two sources: (i) e+e- annihilation cross
sections and (ii) hadronic tau decays. Recently results with comparable
precision have become available from CMD-2 on one side, and ALEPH, CLEO and
OPAL on the other. The comparison of the respective spectral functions involves
a correction from isospin-breaking effects, which is evaluated. After the
correction it is found that the dominant pi+pi- spectral functions do not agree
within experimental and theoretical uncertainties. Some disagreement is also
found for the 4pi spectral functions. The consequences of these discrepancies
for vacuum polarization calculations are presented, with the emphasis on the
muon anomalous magnetic moment. Adding quadratically experimental and
theoretical uncertainties, we find that the full Standard Model prediction of
a_mu deviates from the recent BNL measurement at the level of 3.0 (e+e--based)
and 0.9 (tau-based) standard deviations.
