  We report the discovery of millisecond oscillations in an X-ray burst from
the X-ray transient SAX J1748.9-2021 in the globular cluster NGC 6440.
Oscillations at a frequency of 409.7 +/- 0.3 Hz were present in one of nine
X-ray bursts observed with the Proportional Counter Array on the Rossi X-ray
Timing Explorer during the outburst which occurred in 2001. The burst was one
of the two dimmest and had the longest duration and decay time. The average
peak luminosity of two bursts showing radius expansion is (3.6 +/- 0.4) x 10^38
erg/s, consistent with the Eddington luminosity for a 1.4 solar mass and 10 km
radius neutron star burning hydrogen-poor matter. We speculate that the
dichotomy observed between sources with burst oscillations at once versus twice
the frequency difference of kHz quasiperiodic oscillations in the persistent
emission may be related to the magnetic field geometry of the neutron stars.
