Importing DV files into iMovie can take quite a while, even though iMovie uses DV as its native format, and no conversion is needed. A quicker way to do this is to create a new iMovie project and the close it again. Locate the project file you just created, Control-click it and choose Show Package Contents. This will open in a new window with some folders. Drag your DV footage into the folder named Media and close the window. Now, double-click the iMovie project file to open it. iMovie will open and then prompt you about some stray file; select View Trash. Then simply drag the clips to the media pane. This is very useful for users who have non-mini-DV camcorders who have to manually convert and then import their clips.
kirkmc adds: There's a similar hint here, but the new hint discusses importing directly from a camera, rather than importing varied format videos. It seems different enough to warrant its own inclusion.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2007041600452832