Georgios, they are keeping the V8s until 2013 simply because that's what they started with. It wouldn't make much sense for them to change the engine regulations willy-nilly when they have costs to keep track of, plus it gives engine manufacturers more time to prepare for the upcoming regulations. And if a V6 has a longer engine life than a V8, then if they were implemented in F1 the 8-engines-a-year rule would become nearly obsolete...
I know but Mr.Bernie want's to make F1 green (He said so on one of the news of Eurosport during the January of 2009, he might have changed him mind but anyhow) so I would asume that using engines that hold for a longer time but still be as fast as the V8's would be something he would go for.