My question was also posted on slashdot (http://ask.slashdot.org/askslashdot/08/08/11/1944207.shtml).
I just want to add one more thing to the "productivity discussion":
It would only make sense to compare interns and "experienced employees" over the same period of time (3 months) AND doing the same things. But interns are usually assigned much simpler tasks that can be picked up easily. So in terms of productivity interns are definitely not at 20 per cent of somebody doing the same thing as the intern but who is not called an "intern". I hope that it is clear now what I actually wanted to say.