Chris, I've got a couple HD video cameras and even on top quality setting, I can't get license numbers. I carry a small digital audio recorder attached to the velmobile to record that information. On the camera side, I've been happy with the quality of the ContourHD line, although mine are 720p models and they now have a 1080p, which may have the necessary resolution to get plate numbers. As Keri noted, playback is pretty challenging, although I've considered adding a monitor to the payload I now carry on my commutes. There are smartphones that take the micro SDHC cards, aren't there?
I also picked up some "spycams" that are amazingly compact and provide quite good quality video as well. I had intended to wear one into the local PD for my visit, in case things got stupid, but it went well and I had forgotten the camera anyway. It's called the HD Scorpion Police Security Camera and is barely larger than a flash drive. I lost one of mine and found it again, in my bundle of flash drives, that's how tiny it is. I see the site: http://www.stuntcams.com/shop/
has many different products from which to choose.
I also carry a Canon point-and-shoot for those moments when I have that much more time to yank it out, turn it on, hit the shutter button and get a truly high resolution photograph. One point two second boot time means no waiting for the first photo.