We calculate the leading isospin-violating and electromagnetic corrections
for the decay tau -> pi^0 pi nu_tau at low energies. The corrections are small
but relevant for the inclusion of tau decay data in the determination of
hadronic vacuum polarization especially for the anomalous magnetic moment of
the muon. We show that part of the systematic differences between the measured
form factors in tau -> pi^0 pi nu_tau and e^+ e^- -> pi^+ pi^- is due to
isospin violation .Comment: 12 pages, 2 figures, minor revisions in the text, references added,
version to appear in Physics Letters