Keyfobs are (in most countries) limited to 10mW compared to several watts for cellphones.
You would have to be very lucky, or unlucky, to cause some kind of effect on a computer with a cellphone, but it is possible if there is a cable that happens to be just the right length that it resonates at the cellphone transmit frequency. Usually it's just the monitors that show cellphone signals (and radiate the interference). If the computer system were in a bank, I for one would prefer not to take the chance with it.
Gideon.
