I don't think there is a specific program available, but you might find a general 3D graphics program useful. I've designed a few cakes using SketchUp from Google. You can do different shapes and colours to see what works together etc, I've only just got it, so I'm sure it can do more than that! If there is an actual cake decorating program out there, I'd be interested to know about it too! I often just use good ol' pencil and paper though!