Can't say I'm entirely sure, but there's been at least one large scale military agreement (Re:
Hague Convention) that banned their use by military, so someone's apparently figured it out.
Googling now, 'cause this actually interests me. I'd imagine a fragmentary bullet could manage to put something poisonous on the inside, but how else you'd go about it I'm not sure.
EDIT: Seeing a number of tertiary references to poisoned bullets, but nothing direct yet. Apparently mercury can be used, somehow?
E2: Oh hey,
American Civil War reference: "Some of these parties retain the bullets, which prove beyond doubt that such death-dealing missils were used by the Federal army. Says Mr. Hayden 'The bullet was in two parts, one hollowed out and the other also hollow, being encased in the larger and containing the poison, the latter being loose would slip out and remain in the body or pass through leaving its poison.'"
WW1, I believe: "Moreover, the shrapnel bullets, instead of being smooth, were especially holed and dented. When analysed by a French Government chemist the coloured powder was found to consist of a mixture of white and red phosphorus. The holes in th bullets were designed to carry the chemicals into wounded bodies."
I guess that's enough searching. Poisoned bullets, ladies and gents.