One of the things that I dearly miss on Nseries devices when it comes to video capture is the lack of auto focus in the video mode. Slightly older devices like the N82 have a fixed focus while recording video which is set close and newer ones like the N85, N96, N97 and so on have it set slightly further. This fixed focus limitation also means that making macro videos is virtually impossible.
The N900 fixes this. Like the picture mode, you will be able to use the auto-focus function when you start capturing the video. There is still no continuous auto focus, but that is something you do not require often anyway. Mark from The Nokia Blog has put up this video demonstrating how it works.