Finding an interesting, original application for one of these that is not already done by an existing piece of kit is the hard bit.
All I can come up with is a non-invasive way of making antique clocks like a grandfather clock accurate by mounting a pi-based unit on the pendulum which listens to the clock ticking and an accurate radio time standard like MSF, and by winding itself up and down the pendulum, phase locking it to the standard.
I suspect the Pi might be a bit big though, and too power hungry. Atmel make really nice low-power microcontrollers that would do the job just fine. I have all I need to make it except a nice but not very accurate old clock however!
And I can't see it addressing a mass market either!
pi was ment for schools and learning programming.... it's done the job with me i now have learnt python and regaining intrest in php and mysql again.
just it's good it has GPIO so you can use it for loads of different things.