Yep, Parallels is the problem. Parallels defaults to using a built-in video driver. I've asked on the Parallels forum whether it's possible to have a Windows Virtual machine to use 'real' drivers but they're pretty hopeless at helping there so we'll see.

In the meantime I've installed Windows 7 on Bootcamp and Moi3D is displaying backdrop images.

YAY!

Thanks all!
Sean