Interesting question. I've always taken it to mean a shot of something invigorating, like vitamin B as o'b mentioned above. It would be very interesting to know whether the expression came from innoculations! I would have taken the expression all my life in a completely different way--my interpretation as having been one leaning toward invigoration when, in fact, the expression may have orginally meant more of a deus ex machina slant!

Please, someone, clarify this!