auto focusing Nikon D300  

It could be that the camera is focusing on something other than the actual subject of your shot. Try to push the shot button down half way and let the camera focus, if it is blurry try again and see if you can make it focus on what you are trying to shoot. My Cannon does that sometimes.

When you hold the shutter half-way down the camera will focus. As soon as you've got the photo in focus pressing the shutter all the way down will take the photo. However, if you hold the shutter half-way down, focus, and then let go of the shutter, the auto-focus will search for something else to focus and your image will get blurry!

Sometimes I make the mistake of holding the shutter halfway down to focus, then when I'm ready to take the picture I take my finger off the button and then press down again to take the picture! It the split moment my finger is off the button, the camera tries to re-focus and my image just comes out as a big blur! Could this be your problem as well?