Hi Iain,
It is not possible to customize the UISwitch control using Apple's documented APIs. Some developers have created their own custom switch control, for example:
www.catamount.com/blog/?p=1063
If you are interested in using any of these custom controls, you'll have to create your own custom cell class and subclass it from SCControlCell, then add the custom control to it (only applicable to STV 2.0 and later). You should then normally set a tag for the control and bind a property name to it, using the same technique used to create custom cells in the "Custom Cells App" sample application.
As a final note, you must make sure that the custom control you'll be using is not calling any private APIs (many of them do!), otherwise your app will be immediately rejected by Apple.