A correction to my previous Nikon D5100 post: today I played some more with the video functionality of the Nikon D5100 and you can set the aperture manually for video recording. The trick is that you have to do it before you turn on LiveView when in aperture priority, otherwise the aperture selection will be overwritten. It would be nice if Nikon had a chapter on this in their product manual.
Here is a quick video example of the Nikon D5100 at f/1.4 and f/16:
Sorry about the misinformation.