In previous versions of iMovie, Full Screen playback of a selected clip would play starting from the position of the playhead. In iMovie 2.1 for Mac OS X, playback starts from the beginning of the clip. If no clip is selected, Full Screen playback starts from the beginning of the project.
Solution
Apple is aware of this behavior, which only affects full screen playback. You can preview your clip or project using the Viewer to start from the position of the playhead.