I've seen enough episodes of This Old House, to know that it's a specific piece of hardware that creates the decorative molding.
A quick google brought up this"
http://www.mlcswoodworking.com/shops.../bt_mold2.html
Which has the different bits for the routers depending on the shape you want.
I'd imagine Router's are pretty expensive to buy, but you might be able to rent one if you wanted to do it yourself. Or check with your local home improvement center and see what tehy charge to do it for you.