I'm as skeptical as anyone of Qantas's "explanation", which smacks of legal CYA bullshit, but Mythbusters didn't "prove" anything. The test they did is far from universal. They tested one model of aircraft, with one specific set of instruments in it, with a small fraction of available RF spectrum. (NB: I haven't seen the episode, but I'm certain their test was not remotely universal.)
Furthermore, while Qantas is claiming "instrument" interference and Mythbusters demonstrated in one highly specific (and totally unrelated) case that cell phones did not cause interference with an airplane's instruments, I can state with absolute certainty that in *many* aircraft, GSM-based cell phones can and do cause minor to severe audible interference on the communication radios. In fact, of the aircraft types I've flown, I can only think of one in which I've never heard any cell phone-based interference on the radios, and that's more likely due to my lack of time in that plane (only about 3.5 hours, compared to a hundred or more in other types) than any particular immunity to interference it possesses.
Bottom line: Qantas is searching madly for excuses, but that doesn't mean interference can't be a problem. Airbus planes in particular are known to experience lots of random electrical glitches (all screens in the cockpit going dark, total transient electrical failures, etc.) for no apparent reason, and it's exceedingly likely this was another instance of that.
...or the unqualified son of the chief pilot was in the cockpit and deployed the slats at altitude, like in Michael Crichton's Airframe
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"More software projects have gone awry for lack of calendar time than for all other causes combined." -- Fred Brooks, Jr., _The Mythical Man Month_