Metals have an effective free electron gas which responds to externally applied electric or magnetic fields as a good reflector. (This is why metals are shiny). In a microwave oven, there is a strong periodic field at about 2.2GHz which normally excites vibrations in OH bonds -- thus heating water and oils. However, Metals of the right shape while reflecting the energy can have large potentials on the surface. If two different potentials are close, then like an antenna, the metal sponsors an arc. This can only happen if there are regions with discontinuities in close proximity-- Many microwave ovens have metal frames and/or grates -- these are specially designed to not be effective antennas.

Inside a microwave you have an intense oscillating electromagnetic field. This field will create electric current inside everything that is placed within the "cavity" of the oven. The intensity of the current generated and therefore of the heat liberated in a body will be proportional to the electrical resistance of the body. Insulators (high resistance) will experience fairly low currents and an amount of heat that can be controlled and used for cooking purposes. Metals are much more conductive, and
therefore, the field creates very high currents on them. These currents liberate enormous amount of heat and there you have, the metal melts, sparks, explodes...

Electronmagnetic waves, like those emitted in a microwave oven, cannot penetrate metals, instead inducing surface electrical currents in the metal. Where there is a bend in the metal, the electromagnetic fields are concentrated, and the sharper the bend, the more the electromagnetic fields are contracted. With very sharp corners, such as the tines of a fork or the edges of aluminum foil, the fields can get so concentrated that they are strong enough to strip electrons off of the molecules of the air. This ionization of the air is the spark that you see. How do you think this effect affects the design of lightning rods?

Basically, if you have a piece of metal in the microwave, charges in the metal move around. If there is a part of the metal that is very thin, such as with aluminum foil or a fork, a high voltage could build up that exceeds the breakdown voltage of air and cause a spark.
Here's a related page I found regarding some experiments that people have done with their microwave oven. I don't really recommend trying these at home. Wait until you have your own oven to damage and you pay for your own health insurance!
http://margo.student.utwente.nl/el/microwave/