But now it is.
In SketchUp 7 (which we just released yesterday), Dynamic Components are 3D models that behave like the real-world objects they represent. They make using SketchUp faster and easier than ever, so you can spend less time modeling and more time designing. They're just "smart" versions of regular SketchUp components, so you can use them in any model you're working on.
Check out this video for a basic rundown on Dynamic Components (a picture's worth a thousand words, after all...)
You can use existing Dynamic Components in both (free and Pro) versions of SketchUp, but if you want to make your own, you'll need SketchUp Pro 7. Dynamic Components work by letting you attach attributes (pieces of extra information) to SketchUp groups and components. After that, you can add formulas to create relationships and control how they interact. It's a lot like working with spreadsheets, actually.
And if you're not sure, just download SketchUp Pro 7 to play around with them. You'll find plenty of resources to help you get started: videos, FAQ, self-paced tutorials, working examples and of course, written documentation.