I did check a large taped cardboard box in Dec 2005 at ORD. They didn't bat an eyelid. I stuck around to see what would happen (luggage is examined in full view of passengers at ORD). It went through the X-ray and sniffer machines and was not selected for further examination.

If it had been selected, I assume they would have taped it up...they have lots of TSA tape! If they weren't willing to slit it open and tape it back up, they would presumably have refused to take it at all.