Around 0.6% of all Bitcoin are known to be lost, so with unknown losses it could be 1-2%(https://bitcointalk.org/index.php?topic=7253.120)

Excluding the Pirate scam, 2.9% have been stolen. These are still viable, some are tainted since the receiver address is known. (https://bitcointalk.org/index.php?topic=83794.0)