Thanks to all who have replied, especially those giving legal analysis. I feel like I have a much better understanding of the law now. Sometime after I posted this originally, I ran across this DOJ paper on the issue,
http://assets.sbnation.com/assets/11...ltimore_PD.pdf, which gives some more detailed legal analysis and is pretty consistent with the legal opinions expressed by Karl and SJ.
I'm not planning to video the police whenever I encounter them, but I might video some police encounter with a suspect or if I thought the police were abusing someone. If that happened, I wanted to know what legal ground I stood on if police demanded I hand over my video as evidence for some crime (whether their own or anyone else). Based on what I know now, I think it pretty unlikely the police would actually have the legal authority to take my camera because I doubt exigent circumstance would exist, especially if I volunteered my identity and to preserve the video. But, as SJ notes, exigent circumstances is a pretty fuzzy legal concept and there's no guarantee a judge is going to see it the same way I do.
Also, as Karl notes, there's the law school exam answer to a question and there's the real world. I've perused some police message boards, and the general opinions there seems to be that cops can seize a camera as evidence whenever they feel like it. Some even suggest doing this as a pretext to discourage citizens from filming them. Those opinions I've seen are completely ignorant of the legal analysis, so I would not expect cops to understand the finer points of the law on seizing cameras as evidence.
So, with all that in mind and the fact that I'd prefer not get arrested, I think I'd do the following if I ever found myself in this situation. First, I'd do my best to avoid giving the officer any excuse to arrest me for disorderly conduct or interference by staying out of the way and not yelling or cursing. Second, I'd ask someone else to video the officer demanding I turn over my camera to him. Third, I'd identify myself, promise to preserve the video, and tell the officer I'd be happy to comply with any warrant for my camera (this would bolster a case that there's no exigent circumstances) but until them taking my camera would be a violation of my rights and that I do not voluntarily consent to handing over my property, But, fifth, if the officer says he's ordering me to hand over the camera and insists, then I'm obeying because the exigent circumstance exception is just too gray a concept and I'll likely get arrested if I refuse.
If anyone is curious, here's the original item I read that got me wondering about this issue.
http://photographyisnotacrime.com/20...ras-witnesses/. And, while I was thinking about scenarios with evidence of a bad cop (for example if there were evidence that the cops in the article used excessive force), I was also asking about situations where the video might be evidence of someone else's crime because it's often the case that whenever there's video of a cop using excessive force, the cop probably has an argument that the video is evidence of the suspect resisting or evading arrest or assaulting the cop. So, my legal question was not just limited to evidence of police misbehavior.