I tried it when testing my CarCam P9000 camera when I first got it. I had it powered by a cell phone charger with mini usb cable. But on that camera it took a lot of motion to trigger recording. Pointing out the window, cars passing by and me walking in front of the window didn't trigger the recording, only when I walked really close and filled the view did the recording start. In my testing, by the time the recording started I had already moved out of the scene already so no "evidence" was collected just a video of the empty room.
I think for the price of a dash cam, a more functional setup is to use one of the
D-Link netcams. These record stills and video with motion detection and store to PC or upload to an FTP site. Instructional
video here, see at about 3:22 where they talk about the video recording/upload feature after motion detection. The trade-off then is in resolution. It seems the dash cams have higher resolution than the consumer grade netcams. But at night in pitch black, the D-Link netcams with IR lighting will capture images when there is no light whereas the dash cam will give you a black video. In this case, unlike dash cams, the IR lights work.
I also have PC based webcams with motion detection software (check out
TinCam, its great, now free and creates its own webpage on your webspace to display the motion detected images). For low power I've also used
HP thin clients from eBay that have no moving parts, no harddrive, wifi and run an embedded version of Windows XP. But my latest cameras have been the D-Link netcams, they draw little power and can be placed anywhere there is AC power and setup of the functions is straight forward and can be adjusted by logging into the camera from anywhere in the world. There is a smartphone app that lets you see
and hear in realtime what's going on back home.