I've just come back from Florida, and I was carrying a Fenix P1D-CE, Surefire E1E, Fenix L1T V2.0 and Lummi Raw Ti onto the plane including NiMH, Lithium and Li-ion cells. It is not only a good idea to carry the spare cells in your carry-on luggage, it is ESSENTIAL! The logic behind this is that the cabin crew can access you carry-on baggage using fire-fighting equipment if there ever was a battery-induced fire. If the cells were in your checked baggage, a battery-induced fire could jeapodize the aircraft.