While sliding doors is far from a new concept and will be obsoleted once we see wider support for multiple backgrounds (CSS3), it is still a good one for your bag-of-tricks. The theory is that if we have an extra hook in our markup, we can apply two overlapping background images to a single spot of text. When that text grows or shrinks, these overlapping images and reveal more of themselves creating the illusion it is a single expandable graphic. In this screencast we do a quickly example of how this can work for a button.
*May or may not contain any actual "CSS" or "Tricks".