Mythbusters Episode and FMS

Saw a repeat episode of Mythbusters for the first time last night about
the use of cell phones on an airplane and interference with cockpit
instruments. I know that this was mentioned in a November 2006 thread
briefly but the short of it was that they concluded cell phones really
CAN interfere with the VOR signals.

One of the tests however, was on a Hawker jet with a glass cockpit
(Honeywell I believe) with a FMS. They could not get any frequencies or
amplitude of cellphone signals to budge the instruments like they were
able to on a 1970's era NAV system. From what I understand, the FMS
relies on a number of different inputs to drive its nav systems. If
that's the case, wouldn't it switch to another source if one seemed
unreliable? If it does, I imagine it would be a fundamental flaw in
their experiment. The FBO manager and avionics guy being interviewed
however, gave the nod to the robust wire shielding.

Marco