It's easy to find references to this on the web, so I shall keep my little contribution to this discussion very brief.
In July of 2007 I was visiting a local farmers market and noticed two very similar potted flowers. I decided then to run a little experiment of my own. I brought them home and set them on the balcony, side by side. On a shelf inside I kept two identical containers. One I filled with tap water that had been boiled in an electric kettle and then cooled. The other I filled with tap water that had been microwaved to boiling point and then cooled.
One plant I labeled "E.K. Water" - that is, Electric Kettle Water. The other I labeled "M.W. Water." Each plant was watered at the same time, until the water started to flow from the bottom of the pot. They were given no other food.
Here is a photograph of the two plants the day after I brought them home. In all these photographs the plant that was fed microwaved water is on the right.
That photograph was taken July 13th, 2007. On July 22nd I repotted the plants, and photographed them the next day. One's immediate impression might be that the plant on the right, the one fed microwaved water, was not doing as well. This is because the new buds have not opened - on a closer look you will see that the foliage looks healthy. Here is the photograph taken on July 23rd, 2007.
The experiment continued until August 15th, 2007. Only a month. At that time we went on vacation. The person who was to keep the plants watered did not do a very good job, and both were bone dry when we returned. However you can see from the next photograph that both plants looked vibrant the day before we left.
Granted, this is not much of an experiment. It does show, though, that feeding a plant microwaved water for a month will not kill it.