Another quick tip I haven’t seen yet…
When you’re exporting your iMovie out to your camera roll, you may want to change Auto-Lock to “Never” prior to doing so. It appears that iMovie’s export process doesn’t really ‘freeze state’ or use any of the other semi-fake iOS4 multitasking methods. In other words, if your iPhone goes into sleep state (ala, ‘the display turns off’) while exporting, you have to start all over again.
This can be a real pain if you’re exporting a 5 minute high-def video, since HD exports take at least 2X the video length to completely finish.
Don’t forget to turn Auto-Lock back to something like 1 minute if you want to save the most battery power. So far, I’m not very impressed with the iPhone 4’s advertised superior battery life…but I am comparing it to a Mophi-Juice Pack encased 3GS previously.