edit - NERD-VOL beat me to it 6-7 hours ago.
Stewart-Haas Racing spokesman Mike Arning said Stewart sustained a broken right tibia and fibula in a crash at Southern Iowa Speedway in Oskaloosa, Iowa. He was transported to a local hospital by ambulance, where he underwent surgery.
Stewart will miss at least this weekend's NASCAR Sprint Cup race at Watkins Glen; Arning said a replacement has not been named and it's unknown how long Stewart will be out overall. A test scheduled for Tuesday at Atlanta Motor Speedway has also been canceled.