Scientists have figured out how to focus the energy in a microwave oven more precisely to avoid zapping Wi-Fi networks: The research was intended to improve radar technology, but the University of Michigan team sees great potential in helping microwave oven makers produce Wi-Fi and other 2.4 GHz spread-spectrum technology friendly home appliances.
And it's worth noting here again that microwave ovens do not irradiate food, as in exposing food to radioactive sources to sterilize them. Rather, beams of microwave energy reflected entirely inside the a sealed oven cause the dipole water molecules in food to twist as the energy is switched on and off, causing friction to heat food. (Here's a a whole pile of details on how these ovens work.)
The greatest risk from a microwave oven? Superheating water in a mug. It's very easy to do and extremely dangerous.