I've never seen the flash, but my father swears he saw it once. From Wikipedia:
Green flashes and green rays are rare optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible for a short period of time above the sun, or a green ray shoots up from the sunset point. It is usually observed from a low altitude where there is an unobstructed view of the horizon, such as on the ocean.
Its explanation lies in refraction of light (as in a prism) in the atmosphere and is enhanced by atmospheric inversions. Whilst we would expect to see a blue light, the blue is dispersed (this is why the sky is blue) and only the green light remains visible.
With slight magnification (caution: never point binoculars or a telescope at the sun), a green rim on the top limb of the solar disk can be seen on most clear-day sunsets. However the flash or ray effects require a stronger layering of the atmosphere and a mirage which serves to magnify the green for a fraction of a second to a couple of seconds.
For more see http://en.wikipedia.org/wiki/Green_flash