  The recent discovery of high frequency oscillations in giant flares from SGR
1806-20 and SGR 1900+14 may be the first direct detection of vibrations in a
neutron star crust. If this interpretation is correct it offers a novel means
of testing the neutron star equation of state, crustal breaking strain, and
magnetic field configuration. Using timing data from RHESSI, we have confirmed
the detection of a 92.5 Hz Quasi-Periodic Oscillation (QPO) in the tail of the
SGR 1806-20 giant flare. We also find another, stronger, QPO at higher
energies, at 626.5 Hz. Both QPOs are visible only at particular (but different)
rotational phases, implying an association with a specific area of the neutron
star surface or magnetosphere. At lower frequencies we confirm the detection of
an 18 Hz QPO, at the same rotational phase as the 92.5 Hz QPO, and report the
additional presence of a broad 26 Hz QPO. We are however unable to make a
robust confirmation of the presence of a 30 Hz QPO, despite higher countrates.
We discuss our results in the light of neutron star vibration models.
