Via The Straight Dope, I came across this page which claims that Lord Jeffrey Amherst, commander-in-chief of the Brits in America seriously considered distributing blankets infected with small-pox to Native Americans:

P.S. You will Do well to try to Innoculate the Indians by means of Blanketts, as well as to try Every other method that can serve to Extirpate this Execrable Race. I should be very glad your Scheme for Hunting them Down by Dogs could take Effect, but England is at too great a Distance to think of that at present.

While it is not known if the plan was carried out or not, is this the earliest known example of bacteriological warfare?