Unlocking iMovie '09 & '11
This is a new feature in iMovie '08, and its a gem. You can quickly cut or add frames to a clip in your project by doing the following:
1. Hover your mouse cursor over the clip you want to edit. (If you hover near the beginning of your clip, you will be cutting or adding to the beginning. If you hover near the end of your clip, you will be editing the end.)
2. Use the right or left arrow keys while holding down the Option key.
If you are adding frames, you will see it working as the clip grows beyond the yellow selection outline. If you are cutting frames, the yellow outline will still cover the entire clip.