I wonder if anyone has successfully converted a
thrown-away cell-phone into a spectrum analyser?

We would seem to have all the necessary components
in there for free - keyboard, graphical display,
post-demodulation DSP, Frequency Synthesizer,
RF good up to 1 GHz (2G5Hz if an ex-WCDMA unit)

Here would seem to be an opportunity to equip
all Radio Hams with a reasonably state-of-the-art
piece of test gear, that when coupled with an
IF generator would give us all a network analyser
into the bargain!