Looks like Google should've used BigDecimal!
Try googling this: “1 - 0.9 - 0.1”. Google’s answer: 1 - 0.9 - 0.1 = -2.77555756 × 10-17.
Yahoo gets it right, though.
via Amr Awadallah (who is a Yahoo employee).
(the BigDecimal thing is a joke, btw)