it doesn't really have AF during video. it's a fake "AF" that Nikon uses as a gimmick to make it seem better. Here's why:
On the T2i, you can autofocus during video. However, it involves pressing a button which makes the focus go crazy for a second before becoming focused. The "autofocus" on the D3100 is basically the equivalent of repeatedly pressing that button on the T2i . True video autofocus, especially on a good video camera, usually won't act like that. Much of the time, depending on the shot, you can't even tell a real video camera might be using autofocus. It's very subtle and very fast. Here's an example of the D3100 autofocus and I think you will see what I mean. It's really useless.
to the best of my knowledge, Sony is the only DSLR manufacturer to include true video autofocus as of right now.