That green light clue. Is that the memory card "busy" light? Why don't you try a different SD card and don't forget to format it first. Also, why don't you change the menu to allow it to shoot when there is no card until you get to the bottom of your problem. I don't like that setting, but it might help isolate the problem.
Do you look through the viewfinder with your left eye? If so, you may be bumping the multi-selector with your nose. It is a common problem, though some bodies are more prone to it than others due to the location of the multi-selector. Good Luck and Enjoy your Nikons!
> >Can I lock it at the centre focus point, somehow? > > I guess that would cure the problem, but even if there's no >way to do this, now that I know whats wrong it will be less of >an issue. > >I could do with locking that centre point in tho- I rarely, if >ever, need to change it. I like my subjects in the middle of >the pic!
This is the one complaint I have with the D5100. I haven't found a way to lock it in the center. All my other bodies lock to center. Is this a Nikon oversight or a fault? Other then that the camera is great.