Don't know if I am the last one to figure this out, or if it works in previous versions, but I discovered a great shortcut while working on project the other day. This was a "movie" of photos, and the way I do this is drag in the photos I want to use, and make adjustments to the Ken Burns effect. Then prior to my discovery, I would add in the transitions, clip by clip. Very tedious (and slower in '06 than in '05 on my aging TiBook 1Ghz). Then I discovered that you can select all the clips that should have the same transition, select the transition, and click the Add button, iMovie will add the transition to all the clips simultaneously. No more dragging them one at a time!
It's so much easier that I might just go back and redo a projects I am not happy with!