Sometimes it may appear that the Ken Burns Effect won't stay on. For example, if you turn off the effect and add a photo with no motion to your movie, then select the Ken Burns Effect checkbox, clips that you select in the Photos pane don't show motion in the Preview monitor (even though the checkbox is selected). If you select another photo and drag it to your movie, the Ken Burns Effect checkbox is automatically deselected and the photo is added as a still.
This lack of motion saves your computer's processor time and allows iMovie to perform faster because it doesn't need to create motion where there is none. It also saves you time if you want to add a lot of still images quickly.
If you want to add pan and zoom to a photo after you've added a still image, select the Ken Burns Effect checkbox and set up the pan and zoom effects you want. When you click Apply, the clip renders as usual, and the Ken Burns Effect checkbox stays selected.
This information comes from the Late Breaking News document included with iMovie 3.0.3.