Recently I was asked if you can "backwards save" a PowerPoint 2007 presentation in order to view it in an earlier version of PowerPoint. The quick answer is "yes". One of the Save As options in PowerPoint 2007 is to save it as a PowerPoint 97-2003 presentation, making your presentation compatible with earlier versions of PowerPoint. I find it interesting that Microsoft shows this saving feature as "fully" compatible with earlier versions. This is technically, not the case.
Although you are able to backwards save your PowerPoint 2007 presentation, features you may have used that are new to PowerPoint 2007 will not be available when viewing it as a PowerPoint 2003 or earlier version. This includes such features as design themes or SmartArt graphics, to name a few.
If you still have your copy of an earlier version of PowerPoint, in addition to PowerPoint 2007, you might think about installing both versions of PowerPoint on the same computer to make sure all features that you do use are available to those clients who only have an earlier version. However, to be clear, I am not suggesting that anyone purchase two versions of this (or any other) program.