Heard an Alaska FO used his cell shortly after the Capt began taxiing away from the gate. Capt requested taxi back to the gate. Upon arrival at the gate, Capt informed the FO he would not be operating as a crewmember on the flight, and a new FO will be needed.

It's a brave new distracted world. On my MIL flights, I always announce "cell phones off" to the crew on the Eng Start Checklist.