  An obstruction to the implementation of spatially flat
Painleve-Gullstrand(PG) slicings is demonstrated, and explicitly discussed for
Reissner-Nordstrom and Schwarzschild-anti-deSitter spacetimes. Generalizations
of PG slicings which are not spatially flat but which remain regular at the
horizons are introduced. These metrics can be obtained from standard
spherically symmetric metrics by physical Lorentz boosts. With these
generalized PG metrics, problematic contributions to the imaginary part of the
action in the Parikh-Wilczek derivation of Hawking radiation due to the
obstruction can be avoided.
