Like many English-speakers, I hesitate when faced with the necessity of discussing a single one of those dotted things that usually come in pairs; I say “die,” but I feel funny about it. Jonathon at Arrant Pedantry has a nice post on the topic, explaining how the plural -s went from voiceless to voiced but “remained voiceless in dice. Why?”
Well, apparently because people had stopped thinking of it as a plural and started thinking of it as a mass noun, much like corn and rice, so they stopped seeing the s sound on the end as the plural marker and started perceiving it as simply part of the word. Singular dice can be found back to the late 1300s, and when the sound change came along in the 1500s and voiced most plural -s endings, dice was left behind, with its spelling altered to show that it was unequivocally voiceless.
He discusses truce, bodice, pence, and other words, and in the comment thread John Cowan quotes a great Ambrose Bierce definition. Check it out. (Via Stan Carey’s Link love: language (41), which links to lots of other great stuff as well.)