Steve Carlip <carlip@dirac.ucdavis.edu>

In a classical point of view, this question is based on an incorrect
picture of gravity.  Gravity is just the manifestation of spacetime
curvature, and a black hole is just a certain very steep puckering
that captures anything that comes too closely.  Ripples in the
curvature travel along in small undulatory packs (radiation---see
D.05), but these are an optional addition to the gravitation that is
already around.  In particular, black holes don't need to radiate to
have the fields that they do.  Once formed, they and their gravity
just are.

In a quantum point of view, though, it's a good question.  We don't
yet have a good quantum theory of gravity, and it's risky to predict
what such a theory will look like.  But we do have a good theory of
quantum electrodynamics, so let's ask the same question for a charged
black hole: how can a such an object attract or repel other charged
objects if photons can't escape from the event horizon?

The key point is that electromagnetic interactions (and gravity, if
quantum gravity ends up looking like quantum electrodynamics) are 
mediated by the exchange of *virtual* particles.  This allows a
standard loophole: virtual particles can pretty much "do" whatever they 
like, including travelling faster than light, so long as they disappear 
before they violate the Heisenberg uncertainty principle.

The black hole event horizon is where normal matter (and forces) must
exceed the speed of light in order to escape, and thus are trapped.
The horizon is meaningless to a virtual particle with enough speed.
In particular, a charged black hole is a source of virtual photons
that can then do their usual virtual business with the rest of the
universe.  Once again, we don't know for sure that quantum gravity
will have a description in terms of gravitons, but if it does, the
same loophole will apply---gravitational attraction will be mediated
by virtual gravitons, which are free to ignore a black hole event
horizon.
 
See R Feynman QED (Princeton, ???) for the best nontechnical account
of how virtual photon exchange manifests itself as long range
electrical forces.