I had a problem using my NExus card at JFK. They kept on insisting it was not a valid ID and I should have a DL. The supervisor finally let me go after producing a credit card (though he said, I should get one with a photo on it)!

It is an acceptable form of ID and is even listed on their website. For my travels, I have now printed this page out, but its stupid for the passengers to educate the TSA on how to do their job!