When iMovie produces HD files, the resolution that is rendered is somewhat lower than 1280 by 720. Steve Jobs has said that the native resolution of some HD cameras in the market is low enough to state that 922 by 518 movies produced by the new version of iMovie as ‘good enough.’
I have an Panasonic HVX200-E camera. It records DVCPROHD files onto P2 chips at 1080i25. It currently costs $4-5K. The native resolution is well below 1920 by 1080.
Panasonic say that you cannot have a small camera without having a small chip for the image to be recorded on. If you have a small chip, the size of the pixels recording the image gets smaller. Panasonic say that if the pixels get too small, then in low light situations, not enough photons will reach the pixels because the pixels are too small.
Here is a white paper that explains what the native resolution of their HVX200 camera is.
Here is a posting in the DVX User forum that goes some way to explain how you get 1080 vertical pixel images from from a chip with 540 vertical pixels.
Understanding exactly how the the CCD in the camera used to capture your images helps a great deal when designing a workflow that involves footage that needs keying.