Beowulf: Brad DeLong (comment)
"(k) The Secretary may mint and issue platinum bullion coins and proof platinum coins in accordance with such specifications, designs, varieties, quantities, denominations, and inscriptions as the Secretary, in the Secretary’s discretion, may prescribe from time to time. 31 USC 5112(k)" (original).
NOTE Hat tip: Cullen Roche. As of 2013-01-05, this is the first mention of the Proof Platinum Coin concept.