  Recent measurements of the branching fraction for D_s -> l nu disagree with
the Standard Model expectation, which relies on calculations of $f_{D_s}$ from
lattice QCD. This paper uses recent preliminary measurements from CLEO and a
new preliminary lattice-QCD result from this conference to update the
significance of the discrepancy. The "f_{D_s} puzzle" stands now at 3.5sigma,
with sigma predominantly from the statistical uncertainty of the experiments.
New physics scenarios that could solve the puzzle would also lead to
non-Standard amplitudes mediating the semileptonic decays D -> K l nu. This
paper shows where the new amplitudes enter the differential rate and outlines
where lattice QCD calculations are needed to confront recent and forthcoming
measurements.

High Energy Physics - Lattice;High Energy Physics - Phenomenology