Unlimited portable storage - the dream of every serious computer user, especially the mobile ones. CDRW's improved the world vastly, and DVD+/-RW is continuing in that vein - but there are downsides. Discs are damageable, don't handle direct sunlight well, and still cap out around 5GB of storage, max.
USB keys are growing in size, with 1 GB seen more often these days. Portable harddrives have also been around for ages, including USB-interfaced drives... but the implementations always seems a bit lacking.
I think that a better solution would be something along these lines...
An external USB 2 device containing three (3) harddrives, each 200+ GB or so, fashioned into a "brick"-like device. It would have an easily removeable cover for swapping of failed HDD's, an integrated power supply, and a hardware raid controller. It would also have a short-range WiFi nic and the ability to attach to it as a very simple network attached storage (NAS) device, and possible bluetooth as well. USB2 and firewire would also be a must-have.
A standard laptop-style external power transformer should do the trick, and would save some weight & bulk for users who port it between a small set of locations regularly (and therefore have multiple power cords).
Idea is, 200 GB of reliable storage, in a small profile (much smaller than the stack of DVD's + writer that would be equivalent). The multiple methods of attachment provide significant flexibility, basically a swiss-army knife of volume storage.
I'd also like to see a lockable cover using a standard roundbarrel computer key, and the ability to "lock down" the device to prevent unauthorized data retrieval - though a true security paradigm would require a lot of additional thought.