I just came back from Aruba (beautiful place btw) and the entire island gets its water from a desalination plant that has been there for decades. Not only is it enough to support the island's population of 100,000, but the year - round tourists as well. My question is, are there any plans to bring desalination technology to the most heavily traveled/populated Greek islands? Aruba's yearly precipitation is about the same or even a little less than most of the driest Greek islands. I wonder why this hasn't been implemented yet.

Probably because there isn't a need.
They are only generally built when there is an extreme drought and in areas with little rainfall.
Cyprus currently is in a prolonged drought, although they have desalination plants.
Greece is actually shipping over drinking water daily at the moment.

As far as i am concerned such plants are active in some islands.

I didn't know that, interesting. Any idea on their capacity?

50,000 cubic meters of water will be delivered to Cyprus from Greece every day, for six months. Costing 45million euro.

They have two desalination plants in Cyprus, currently being expanded apparently.
It costs 1euro for every cubic meter of water from them.
The water shipments are costing 5euro per cubic meter, mostly due to transport costs. Greece is only charging 0.67euro per cubic meter.

A mobile plant will be ready in October, and another in 2009. They are also planning to build a large, permanent desalination plant by 2011.
The idea is to eventually use renewable energy(solar) to power them.

While I suppose that there isn't an immediate need for desalination plants in certain islands, I believe that we should provide some relief to our water supplies by extracting it from the sea. That water from the sea will never run out, while depleting water tables is a real possibility. Isn't the mainland already shipping water to the islands, especially in the summer months?

I checked the water capacity for Athens at the EYDAP site for the early 1990's, and they were dangerously low. The last thing we want is to be caught between shipping water off the mainland when the mainland is suffering from water shortages itself. It won't hurt to prepare for any future droughts that may come about.

In Chios there are 4 new plants which operate since 2006 and produce 3500 cubic meters per day, also in Ios there is an active plant since 2000 and in Santorini, Mykonos and Syros new plants were finished this year and will be used during the summer.
Generally a good solution for the islands at summer, that water consumption is increased due to tourists arrival, is the mobile desalination plants that were developed by Greek scientists in Aegean university and which operate with aeloic energy.

Also in Crete in order to finally provide Heraklion with drinkable water they will construct a plant to desalinate the water of a nearby river that has subsaline water.