I have a 7-month old D5000. Occasionally the first picture I take after power-up is completely blown out; all other pictures taken at the same settings are OK. Looking at the metadata, I see that the aperture was set to wide open on the first picture. This happened with two lenses, in aperture priority mode; it happened twice with the camera on a tripod - I didn't touch anything but the cable release. The lighting conditions didn't change. It happened with two different AF-S G lenses: 35mm f1.8 and 18-70mm. Has anybody else come across this?
Welcome to Nikonians! Sorry to hear of your problem. Do you have the aperture set wide open in Aperture priority mode? In other words is the Aperture in the second image the same as the first? If the Aperture is changing without changing the settings, it could be the stop down mechanism in the camera is sticking. Nikon Service should have no problem getting it fixed under warranty. Good Luck and Enjoy your Nikons!
If that is the case, a sticking stop down mechanism in the body could be the cause of your problem. Since you said it happens with both lenses should eliminate the likelyhood of a sticking aperture diaphragm in the lens. Good Luck and Wnjoy your Nikons!