The ink cartridge bombs discovered on several flights in recent days contained SIM cards and circuitry which may have been intended to serve as a trigger mechanism. The electronics in the bombs shipped from Yemen are merely timers, and could not have been used as call-activated triggers due to the technological limitations posed by distance and altitude, but it's not impossible. Terrorists could not have used a regular cellphone to call an airplane-borne bomb because it probably wouldn't be able to reach a tower that could bounce the signal properly.
That being said, in-flight Wi-Fi gives them more options, such as Voice over Internet Protocol (VoIP), for contacting a device on an aircraft. Alternatively, suicide bombers could possibly call a bomb in the luggage compartment of the plane. Other experts argue that "the use of mobile phones on planes does not constitute any additional security threat."
Foiled terrorist plots often end with stricter security procedures, but cellphone use and in-flight Wi-Fi are arguably much more important to us in today's society than not being able to bring fake pocket knives on board. Hopefully, security experts will figure out a way to keep the bad guys at bay while the good guys can continue to enjoy the luxuries of the modern world.