Has anyone got any other ideas where this tech would be a positive addition to a tablet?

Obvious use to me would be for on-screen keyboard feedback. I think it'd be a positive step to be able to “feel” the keys under your fingers - maybe even to be able to assign different “feel” to the different types of key. For example, the alphabetic keys could be rough, the numeric keys slippery, etc.

Interest though will depend on how expensive this is to add to a device. I can't see it popping up on the budget tablets though.

This is going to be huge for music production. Producers and music tech companies were quick to adapt to tablet technology anyway, with many hardware devices having a connector port to incorporate an iPad, but the ability of a surface to provide the illusion of edges on controls is going to be dramatic (plus, of course, the ability to take on different illusory layouts at the press of a button).