I've just started dabbling in shooting HD video with the new HD-DSLR cameras. On a recent kayaking shoot, I grabbed a Nikon D300s to shoot some behind the scenes footage and to start familiarizing myself with the nuances of the video functions on a DSLR.
While I won't go into everything I learned over the course of the day, I will bring up one VERY IMPORTANT note I discovered about the limitations of shooting with these cameras. Due to the small sensor size and a lack of any internal cooling system, it is possible to fry your sensor due to overuse. This is something that I was unaware of myself. Even upon returning home, a quick google search didn't return a lot about this phenomena.
In short, long video clips or even leaving the camera in live view mode for an extended period of time will begin to heat up the image sensor. Once that sensor gets hot, it will start corrupting files. Keep shooting and you can actually damage the sensor beyond repair. Fortunately for me, I only corrupted a handful of files at the end of the day without doing any irrevocable damage to the sensor.
I talked to my camera guys at Glazers Camera in Seattle and talked to them about this. Apparently, this is something common to all the HD DSLR's. The small sensor just wasn't built to shoot for extended periods of time. Given the newness of the technology, it is something that the manufacturers haven't yet addressed. I've since heard of some people wrapping their body with gel cold packs to try and stem this issue, but this seems like a horrible idea to me as the condensation will probably do more damage in the long run.
Apparently I'm the first person to come into Glazers with this problem, but they are expecting a rash of similar stories as this new technology is taken out into the field during the hot summer months.
So, I have no answers or sure fire solutions to provide here... just a warning to all the shooters out there. If you're in desperate need of shooting longer video clips, I'd suggest the RED cameras. Their larger sensor isn't prone to overheating apparently.