I have been using my Global Entry card as ID at TSA checkpoints. Yesterday at SAN Terminal 1 I handed ID checker my GE card and boarding pass. He looks at it and said "What is this for?" I told him it was my ID so that I could clear security. He then asked for driver's license. Told him I didn't have it with me, but figured an ID issued by DHS would be acceptable. He clearly was not happy with me, but let me go with no further comment.