According to the car blogs like Jalopnik, GM refuses to release the Caprice to the public, even though it makes no sense (how can you use it as an unmarked car when its easily identified as a cop car since only the cops use it?). The car is supposed to have similar underpinnings to the Pontiac G8, and Australian roots, I think they will release it to the public in due time, they might not think they have the ability, especially because they are under so much public and governmental scrutiny and because its a foreign made/designed gas guzzler.
2004 Chevy Impala "9C1"
3.8 Liters of 'Merican Pushrod Power
56,426 miles as of 7/1/2012