iLife 2008 iMovie is so unfinished that there's no help entry for "join clip". "Join Clip" is the name of an item on the Edit menu.
iMovie 2008 would have been ready around October. Yech.
OS X 10.5, btw, won't be ready until April 2008. My prediction.
Update: The string "join clip" doesn't appear in the PDF tutorial either. It's as though the feature wasn't implemented, but Apple forgot to remove it from the menus. I'm starting to feel like the Emperor has no clothes and I'm the only one who's saying anything ...
Update: I figured out by experimentation what split clip does. If you move a clip to a project, then click and drag to select a portion of the clip, you can then split that portion out. If I could ever figure out how to select more than one clip at a time I could probably join them, but again the help file is useless.