I took a couple of movies the last couple of days with the D90. Since you do it from live view, and focus from there first, I was also trying that out. I must say, it kinda sucked. I was indoors and was using f/2.8, but the focus was the slowest thing ever! I was pretty disappointed. I will have to try it outside sometime to see if it is better with more light, but so far, I am not impressed.
There were several times that I would try to focus and it would miss completely. I would then go out of live view, focus, then pop back in. It would still move around a bit, but would find it eventually. Then when you take a picture, it doesn’t happen right away. The mirror flips back, then the picture happens normally, then the mirror flips up again. Slow. Not a way to take pictures of kids.
The movies turned out OK, when I got the focus right. It would probably be easier with a smaller f-stop, but that is all I could work with indoors. It is nice to have something that takes video though.