I have been through lots of input devices for my HTPC with varying success... and I've just bought an "Accuratus toughball" keyboard & trackball wireless device.
Its absolutely fanstastic... far better than keyboards with touchpad input devices. The finish is top quality and doesnt look out of place at all in my front room.
Once the mouse movement speed has been turned down (to about 15%) then the trackball is very responsive and accurate.
I'd been starting to think that using a PC as a Home Theatre device was going to be really troublesome from an input point of view but this does the job fantastically.
If anyone is struggling to find something suitable which the missus can use without any fuss, this will do the job nicely.