The impact on radar is indeed significant and presents the single biggest challenge to wind farm proliferation in the UK. The turbines are clearly visible to radar, as the blades move at speeds that fall within the detection thresholds. Already there have been numerous public enquiries dealing specifically with wind versus aviation. The spread is significant, Defence Estates deal with in excess of 1000 enquiries per year on behalf of MoD and I don't know of any airports who are not currently dealing with a wind application. The flip side is that generally the wind energy guys are pretty good and willing to listen to the genuine concerns of airports.
As far as obstacles are concerned, the point about lighting is a good one and something I was discussing with a developer last week. They only tend to be lit when in the vicinity of an airfield (and even then only when the airfield insist). However, they are on charts so no sympathy to any numpty who elects to fly into one (although I'd love to be there to watch)
