In one of the collections of Ron Cobb's fantastic artwork (I believe it was "The Art of Alien" but I could be mistaken) there is a device which looks a bit like a flashlight labelled "datastick". As far as I know, the device never appeared in the movie.
I've been thinking about the Datastick ever since. To me, it's essentially a smart storage device (not quite a computer) which serves as an interface between anything that creates or consumes data. Because it isn't itself a computer with a UI it can be cheap and rugged. It's been a considerable source of disappointment to me that many companies build devices which are tantalizingly close to being a Datastick, but none of which are as useful, versatile, or robust.
An iPod could be a datastick. It has a standardized hardware interface and decent storage. It would be more compelling if you could replace its battery and/or plug in more storage.
PDAs and Cellphones
I group these two because, today, any decent PDA is a cellphone and any decent cellphone is a PDA. Sadly, these many of these devices don't yet support decent mass storage options (e.g. normal sized SD cards), standardized interfaces, and they all have too much computer functionality to be cheap and not enough to be useful. (I'd make the computer an accessory.)
So that's the Datastick. I think there's a huge potential market, although the first one will need to pretend to be a PDA/cellphone/iPod to get traction.