http://www.tsa.gov/what_we_do/layers/twic/index.shtm
My husband was required to get one of these cards for work - with his license he would have access to secure areas of vessels that go in and out of US Ports.
In the FAQ's:
Is TWIC an acceptable form of ID at an airport screening checkpoint?
Yes, the TWIC card is an approved government issued photo ID.
It has an RFID chip in the freaking thing - it requires massive background checks. It is OWNED by the TSA.
Not one TSA "agent" that has reviewed it has ever known what it is. They've ALWAYS demanded additional identification. They suck.