actually microwaved water will frequently be above boiling--microwaving lets the water remain still enough for it to get superheated. when you disturb it (with a teabag, spoon, etc.) it will start bubbling wildly (and quite possibly give you a nasty scald)
It depends on what you have the water in. A very smooth-sided container with very pure water has few nucleation sites and so the water is much more likely to become superheated, while a beaten up mug and standard tap water is less likely to superheat without boiling.
happens to my tea all the time--i don't know if it's actually superheated in the sense of being >212, but it sits perfectly still in the mug until i drop the bag in, at which point it goes nuts with the bubbles.
