Requirements:
- iPhone (jailbroken)
- iFiles (software for acessing files on your iPhone)
- Suitable HD video for importing
The critical software here you must have is iFiles. This is the only software I have found that can add the necessary metadata and files necessary to get your video listed on your iPhone in the Camera Roll where you can access it from other apps on your phone. You will need to install iFiles using Cydia or your chosen method of installing apps on a jailbroken iPhone.
I have not done exhaustive testing on what videos are accepted - but in my testing I have found your video must have the following specs:
- Format: H.264 (MPEG-4)
- Dimensions: 1280x720
- Container Format: MOV
- Codec ID: mp42
- Format Profile: Main@L3.1 (very important)
- File Extensions: .mov (very important)
When creating the video -- you always want to export using the 'Custom' QuickTime dialog in order to access the functions you need. Make sure to export the video as H.264 (MPEG-4) at 1280x720, and very important you access the 'Video Options' dialog and make sure the profile is set to MAIN.
Once you have the software installed and your video created you are ready to import HD video to your iPhone.
First, launch your chosen method of file transfer (iPhone Explorer, Phone Disk, iFiles) and mount your iPhone ready for transfer. Where you place the file on your iPhone is not critical as it's a temp location and you can delete the file after importing. For ease of use I might recommend creating a new folder on your iPhone using the transfer software in the location near DCIM/100APPLE, which is where your other images and videos are stored. Transfer the video to your desired place on the iPhone. Remember the location where you imported to.
Once your video has been transferred, launch iFiles on your iPhone and navigate to the location in the filesystem where you imported your video. If your video does not show up on your iPhone you probably did not follow the specs above. Once you locate your video in iFiles, click the blue arrow to the right of your video (if you click in the wrong place the video will just start playing), to access the file attributes. Once you have the File Attributes open for your video, scroll to the very bottom and you should see a button labeled 'Add To Camera Roll.' Click that button and you will get a confirmation (if your video is within specs) or a failure notice that your video has been added to the Camera Roll.
Your video is now listed in the main Camera Roll and available systemwide on your iPhone. Use it in iMovie, watch it with the video player, just do whatever you normally would do with the video.
Notes -- this hint was developed using an iPhone 4. Your mileage may vary if using a 3G or 3GS iPhone... let us know how it works if you try it on one of those phones.
[crarko adds: I haven't tested this one. Jailbreak your phone at your own risk; as noted the methods available to do so are published on the web. I personally don't do it.]

