In recent days I've been asked a couple of times about accessories to make a DSLR into a pro video rig, plus I noticed a post by another acquaintance on a forum asking the same question. It seems to be the question of the week.
You won't like my answer (nor will the camera companies, who've been spending most of their engineering efforts on adding video to still cameras). The short answer is this: you can do it, but you'll spend more money making a fish that can fly than just buying a bird in the first place.
(Most, and certainly all Nikon) DSLRs aren't ergonomically designed to be held for video. That's because you need to see the color LCD, as the normal viewfinder is now inactive. Yes, I know there are some cameras, such as the GH1/2 and the recent Sony DSLRs that get around this limitation. But with them you still find things aren't perfect: videographers have long been two-eyed operators because they need to see what's happening outside the frame in order to anticipate changes in framing. In my videography days, that meant camera on the shoulder with my right eye in an EVF cup and my left eye open and not blocked by a camera partially in front of it.
In the case of the Nikon and Canon DSLRs, the minute you want to record handheld video you're going to discover that you want bars or a gimbal rig to hold it securely in front of you. Ka-ching. Of course, you can't always see the small, built-in color LCD so well, so you either purchase a hood for it or a larger positionable LCD. Ka-ching.
Next, you discover that the built-in mic is picking up camera handling sounds and not particularly good. So you invest in a shotgun mic with muff. Ka-ching. Oh, and the camera's audio input isn't exactly all that great, giving you a crude (at best) recording level setting and no way to monitor it. So you buy a pre-amp and headphones. Ka-ching.
The Ka-chings start adding up. Pretty soon you've spent more on making your DSLR capable of being used for handheld and remote video than it would have cost to buy a dedicated video camera that doesn't have those blind spots. To what end? Oh, right, Hollywood-like bokeh and better low light capabilities. Of course, to get that Hollywood-like bokeh you've also added an expensive fast aperture manual focus lens (okay, maybe you already owned that, though on the crop sensor DSLRs your 28mm f/1.4 is no longer 28mm equivalent ;~). As for the low-light advantage, I'm not sure that it's all there any more, as current video cameras are fairly decent. Sure, a D3s really shines at video in low light (though it will be 720P/24, which isn't exactly the highest resolution or broadcast frame rate), but it sucks at almost everything else video-wise. I'd rather spend the time and energy putting a little more light on the subject and using the right equipment.
Plus, with the Sony NEX-VG10 and the Panasonic AG-AF100 we're now seeing large sensor video cameras in the US$1500-5000 range, and I'm sure we'll see more. One only has to use one of them for a few hours to see just how much kludge we're tolerating with DSLR video.
Don't get me wrong. I've come around from my "no video in a DSLR" position. First of all, it's too late: the cat's out of the bag. But second, it is nice to have the flexibility to capture some video when my DSLR is all I have handy. But I'm not recording video with a DSLR any more. I've switched to a video camera for video. A large sensor one, to be sure, but still, a camera designed for video. It's just a better choice.