People keep doing this sort of thing. IBM had Metacard, there was Oqo, and wasn't there a Palm device which was viewed as mostly a media storage device?
Why not go ahead and add a touch screen (and a stylus) so that one can use it w/o needing to cable up?
I use a Toshiba Encore 2 Write 10 as my main system in pretty much this way, connecting when I don't wish to use the touch screen / stylus to a full-size keyboard and monitor (fortunately, Toshiba provided an adapter for the single USB port which affords two connections, one for charging, one for devices).
The functionality I'd really like to see is this sort of thing done as an iPhone / iPod Touch sized unit ---- Apple could take their laptop, make the trackpad a removable unit which was exactly the size of an iPhone/iPod Touch, and one could replace the trackpad w/ the portable device which would then function as a customizable trackpad and which would load the user directory and backup the portable device.