Apple today released iMovie HD 5.0.1 as a 2.4 MB download via Software Update. The update fixes unspecified audio and video synchronization issues. According to Apple, you should apply the patch if your iMovie HD projects include "DV Widescreen assets [16:9 footage], titles, transitions, or video effects" ...which entails pretty much any iMovie project.
Update: The update also seems to have solved a problem many people were having with using iMovie's pass-through mode. See this thread at Apple's iMovie HD discussion forum for more details.