The full screen preview in iMovie '08 is excellent. Here are a few quick tips on drawing out all of its power.

You can enter full screen mode either by selecting the "Play Full Screen" from the View menu, or by clicking the full screen button below your project or event footage.

The button you want is the darkened rectangle with the arrow inside it. There are two of these, one under your project and the other under your Event Browser.

Once you are in full screen mode, you will immediately notice that if you move your mouse around, a clip navigation shows up across the bottom of the screen.
(Click to enlarge.)

This navigation bar will ignore your mouse movements until you click on it. Once you have clicked on it, dragging your mouse across the bar will scrub just like it does when in the normal iMovie window.

If you have scrubbed to a given point in full screen mode, and want to begin normal playback from that point, just hit the space bar. (Clicking a second time on the navigation bar does not disable the scrubbing ability.) Alternatively, you can move your mouse cursor straight up so it no longer runs across the clips in the navigation bar, then move your cursor over to click the play button.

One other really nice feature is being able to restrict full screen playback to the correct resolution of your footage. Because most of us are editing footage that is lower resolution than our computer displays, a full screen preview will make your footage look worse because it upscales it to fill the screen. If you want to see what your footage looks like in its natural dimensions, go into the iMovie preferences and select "Actual Size" from the drop-down menu selecting full screen playback size.

Now your footage will display at its actual quality.

The more I delve into iMovie '08, the more hidden gems I find. If you have found any hidden gems, please send them my way or share them in the comments.