I think that metal simply reflects the microwaves so provided that there is something capable of absorbing them and converting them to heat I don't think there is a problem per se. My microwave has an aluminium paddle in the top which rotates and presumably bounces the energy where it wouldn't otherwise have gone. I stick my metal butter dish in for a few seconds, to soften the butter, with no bad results. I think the problem is with complete metal rings such as the surface of CDs and the gold patterns on cups. In those special situations you are inducing high voltages into a continuous circuit with interesting results.
Nick