One of the new features to Photoshop CS4 is Content Aware Scaling. This new feature allows a user to take an image and resize it without losing the key parts of a photo. It helps protect skin tone areas and you can also create an alpha channel so that certain areas are protected before the resizing. You really need to see this feature in action to appreciate it.
This tutorial will teach you what Content Aware Scaling is, how it works and how it can help you expand what you can achieve using Photoshop.