I'm currently working on a fun little prank where the microwave will blast dub step after being turned on (OK, well a pair of speakers connected to an arduino with MP3 shield). I'm just missing the proper sensor to trigger the system.
I tried experimenting with the 30A non-invasive current sensor on Spark fun attached to one of the two internal wires of the power cord. The output was too small to read (after factoring in noise) as the current drawn by the microwave is 6A@120VAC. I've also tried using a kill-a-watt's opamp output as per ladyada's tweet-a-watt project but I have a slightly different model and forsome reason, the baseline output is 2.300 vvolts... rising to 2.306 with the microwave on. Also I ended up killing the whole unit somehow.
What other suggestions do you have? Does a microwave output a measurable magnetic field when its on?