You hand draw designs using your mouse, upload your own images and generally get all Tony Hart with a decent set of design tools.
Sadly the service is only available for a single handset, and it's a particularly unexciting one by Nokia's standards. The 7310 Supernova lacks 3G and its camera is a 2MP number with no flash.
Still, it's not often you get to play designer with your mobile, and the finished product is a reasonably affordable £138. Head over here to Nokia Build to get creative.