D5100 Shutter lag problem
I've had the same experience. I took a lot of pictures in the park, then the camera wouldn't focus (could not activate the shutter). Then I focused on something in another area - the shutter performed and all was well with the 5100. A great camera.
Fujifilm F30 Canon G10 Nikon D40 Nikon D5100 55-300mm, 35 f1.8 lens, 18-108mm lens