Well, well...looks like the iPhone 3GS has the capability to do HD recording thanks to its ARM Cortex processor and mods to a few system files. This will allow you to record in 1080×800 vs. 640×480, follow the instructions below on how to enable this on your jailbroken iPhone 3GS.
1. Download the modded system files here (http://www.megaupload.com/?d=4K8K8SO3).
2. Extract the downloaded system files to your desktop.
3. SSH or use iFunBox to access your iPhone file system.
4. Copy N88AP.list to /System/Library/CoreServices/SpringBoard.app
5. Copy AVCapture.plist, CameraRollValidator.plist, and MediaValidator.plist to /System/Library/PrivateFrameworks/Celestial.framework/N88
6. Lastly, reboot your phone and HD recording should be enabled.