I have used a GoPro HD2 as a dash cam, and it can do the job well. I already had the camera for other purposes though, so it was a cost saving to use it, versus buying a new camera entirely. I have the skeleton housing, so I was able to plug in a USB charger, therefore battery was never an issue. I recorded at 960/30, so that it has maximum vertical FOV. So long as I remembered to press record, it was fine.
For others looking at GoPro, it should be noted that both the GoPro3 Silver and Black now support loop recording. Battery life is still a concern, so just buy the Frame mount, and use a USB phone charger to keep it going. They still have the sharpest lenses out there, but that is what you pay for. With built in WiFi, you can use your phone to aim the camera when you set it up.
Also, I would say that clarity and vertical FOV is more important for a dash cam than just arbitrary 1080p resolutions. First, I found 720p to be more than enough to read a license plate if needed. Second, a 4:3 aspect image also tends to be more useful for information gathering purposes. Assuming the same viewing angle in width, a 4:3 image has more viewing top to bottom. This will get things lower on the road as well as higher up (for example, a cube van plate mounted at the top of the box). A dash cam is for evidence gathering, not for making track day performance videos. As such, you need to think a bit differently about the result.