An air compressor is indeed very useful in cleaning the interior of a
piano, and once the action and keys are removed, it blows out a
considerable amount of crud that a vacuum cleaner and soft brush can't
reach.  It is especially good for blowing out the backpost area, and if
the piano is on a tilter, getting bass strings twisted, bridges worked
on, etc., then I can remove the pedal board, point the piano towards a
convenient open doorway, and fire away.  The whole procedure does not
take very long, and helps get rid of felt mites and mold spores.

With that last in mind, I'd advise wearing a dust mask during the
process. (Other people in areas where Hantavirus is a problem might
have some advice to add about cleaning up evidence of mice.)  Two other
suggestions are: keep the pressure at a reasonable level, and don't aim
strong air blasts at damper felts or any other components that can
easily be damaged.

Richard Friedman