err sorry to burst your bubble but yes the N900 does autofocus automatically at the start of a video, especially if you go from locked > recording really quick, like opening the camera lens cover and immediately pressing the record button. the phone does this by default, you will notice it does the exact same thing if you take a picture the same way (ie, really quick)

besides that, i believe a quick forum search will reveal many wishing and some working for a true live video autofocus

till then, try out the Lfocus script if you haven't already.