Give Dailyroads a go, I've just started trying it recently on my Desire (running Leedroid)
https://market.android.com/details?id=com.dailyroads.v
It constantly records, then saves based on input from the motion sensors, you can also touch the screen to retain anyway if you want or need to.
If you press home (instead of back) it carries on running with some buttons overlaying anything else on screen, so you can use gmaps navigation, the MP3 player, etc at the same time
Also IMO the Desire's 720p mode is a bit pants, 800x480 blown up isn't a great deal different and will save a load of space on your SD.