I've tried this over the past few months with a number of different microwaves because we wanted to know if the microwave would function as a Faraday cage... we tried it with every cell phone in the house, we also tried it with the cordless land phone. It didn't work. My conclusion is that a modern microwave will not function as a Faraday cage [Edit for clarity: for communications equipment (WiFi, Cellular, Home Phone etc.)] - I realize that may be painting with a broad brush, perhaps some of them will, but those I tried did not.

If someone has tried this successfully with a modern microwave, it would be useful to know what cellular/radio devices they tested and what make/model/year of microwave they used for their tests.

Microwave ovens most definitely function as faraday cages, however they are specifically designed to attenuate 2.4GHz signals. Anything higher or lower which isn't a harmonic (integer multiple) of 2.4GHz is going to experience much less attenuation.