I had issues with the Mythbusters tests, but your recollection is the opposite of their stated conclusion, which was that some aircraft instruments *can* be affected by RF interference from electronic devices. I'm not sure that a couple of cell phone users is truly an issue for modern aircraft instruments though. I mean, Virgin has WiFi available on their flights for goodness sake.

It's my belief (entirely uninformed belief) that the real reason they prohibit cell phone usage on a flight is the other thing stated in the Mythbusters conclusions: A cell phone that's up high in the air will connect to too many cell towers at the same time. If everyone who got onto airplanes was allowed to fire up up their cell phones, the cell networks would be overloaded by the sheer number of simultaneous cell tower handoffs happening.

I will concede that an airplane full of people using active radio transceivers is going to put out more a lot more RF interference than just a few cell phones would, so I'll give the FAA the benefit of the doubt, assuming that it's not the onesie-twosie cell phones they're prohibiting, but rather the hundreds-of-cell-phones-on-the-same-plane problem they're trying to prevent.