I've followed your tuto to build the theremin with the arduino.
I've bought all the component today.
And it just don't work.
I'm using a mac with the last version of arduino (0021) and a Arduino mega 1280.
I'm using a breadboard and a steel wire (about 1mm length)
I've check the schematic 10 times and everything seem to be alright.
It drives me crazy!
its a problem with the mega 1280
the frequency input is fixed to the t1 timer input on the mega 8/168/328
in the code:
int pinFreq = 5;
the mega 2560 and 1280 has this pin not mapped to the pin header
you can try to solder a thin cable directly to that pin or just use a arduino uno.