There was a joint Hungarian-German study on the subject led by Dr. Gábor Horváth at Eötvös University in Budapest, Hungary.
A team of physicists, troubled by the lack of scientific evidence for the phenomenon, set out to test the theory that water droplets on leaves can act like mini magnifying lenses, focusing the sun's rays and leaving a leaf's surface covered in scorch marks.
Using computer modelling as well as tests on real leaves, the researchers claim to have disproved the theory.
They found that water droplets on a leaf surface were not able to focus the sun's energy sufficiently to damage the leaves before the water evaporated.
It is true that in the perfect circumstances a droplet of water can cause some burn damage but it's only in the case of some tropical plants with hairy leaves and even then it's unlikely to cause too much damage due the hairy leaves tend to shed the water.
However there are other things to keep an eye on as well when you are watering your plants as it is stated on the very same page.
"Drops of acid rain, salty sea or tap water, chlorinated water and concentrated solutions of fertiliser or other chemicals can all cause sunburn-like brown patches.
"Plants could also suffer some kinds of physiological stress from putting cold water onto hot leaves."
That all said and done though. It's still not ideal to water during the midday sun because the plants try to retain as much water as possible and they are pretty much resting (midday depression of photosynthesis can also be examined).
There's a few nice tips on watering plants:
Early is best; dawn to direct sun is ideal. "Never in the evening, unless you like disease,"... Watering in the evening can lead to fungal growth that's damaging to your plants.
Water deeply and infrequently...
Don't wet the leaves when watering plants. Always water at the plant's base.
Water when needed. To figure out if your garden needs watering, use a technique similar to the houseplant test: Push your finger into the soil surrounding your plants. According to OrganicGardening.com, you want the top two or three inches of the soil to be dry, with the soil blow that moist.