iMovie '08 includes an MPEG-2 component that provides import support for certain iMovie '08 compatible MPEG-2 video cameras. This MPEG-2 component does not provide import/export functionality for MPEG-2 files in iMovie '08 or QuickTime.
In order to play back MPEG-2 files in QuickTime, you need the MPEG-2 Playback Component. This component provides playback functionality in QuickTime only and does not provide MPEG-2 export functionality in QuickTime Pro or iMovie.