Vladimir,
To be honest I don't really know. The difficult part about this is that we could put up a survey and ask "what version of JVM do you use" I suspect that 1.8 will be overwhelmingly popular. The problem is the subset of people responding may not be representative of the population that is using it. From my experience working with bigger companies they view JDBC as a simple tool. The metaphor that comes to mind is a car with aftermarket wheels. You can't get any more for the car because you spend 2000 on the wheels. Cars come with wheels; similarly PostgreSQL comes with a JDBC driver. It occurs to me that maven actually exacerbates this as you don't even have to come to the site to get the driver now that you can just add the dependency to your pom and it will be automagically downloaded for you.
While writing this I am thinking that we might get away with saying we are only going to support 3 JVM versions so currently 1.6-1.8, once 1.9 comes out we can drop 1.6, however at the moment this is just a thought, not a policy. Thoughts ?