D3100 autofocus while recording

You will probably get a much better response to a question like this in the dpreview.com forum, it's more suited to this type of camera.

http://forums.dpreview.com/forums/forum.asp?forum=1034

Since the reason that people use DSLRs for video is that they can manually control everything. A guess is that, maybe it isn't supposed to auto focus for video or there is a menu choice to turn it on. What does the manual say?

I have the D7000. If the 3100 auto focuses anything like it you have to push the center button on the AF/M focus lever. The lever should be in the AF position, not the M position.

Once you push and hold that button you use the main command dial to select AF-F, (full-time af). While holding the center button and using the command dial you should be able to toggle between AF-S, (single servo AF) and AF-F. This should all be in your owners manual.