You might have this experience: You open an older presentation in PowerPoint 2007, you add no PowerPoint 2007 features to it, and when you go to save it the Compatibility Checker appears. It tells you that a PowerPoint 2007 feature won't be editable if you save in the old format.
This happens because certain types of formatting and elements are automatically upgraded by PowerPoint 2007 so they'll look as good as PowerPoint can make them.
An example is WordArt. If the older presentation has WordArt in it, the WordArt is upgraded so that it uses the latest effects, new in PowerPoint 2007. The same applies to 3D effects and shadows.
If you save a file that has been upgraded like this to the old format, it won't be editable as a WordArt object anymore but will be converted to a picture. Like the graphic in the earlier example, though, if the WordArt is untouched in the older version of PowerPoint, it's fully intact for editing when the file is reopened in PowerPoint 2007.