I should have known better, it seemed too good to be true and it was. This battery will not charge in the Nikon charger that came with my D40. It must have had some charge when I first got it, because it seemed to work in the camera when I tested it. Of course, when I actually needed to use it it was dead and it won't recharge and it's too late to send it back. Massive fail.
If you've got yourself a D40, then my advice is don't bother getting another battery. I've found that you should be able to take over 1000 pictures on one charge, and I've never been in a situation where I needed more than this between charges.