It sucks taking a digital camera on holiday, because as soon as you fill up your memory card, you have to find a place to burn your photos on CD, or else, lug a laptop around with you.
We already have USB hard drives that you can carry around. We also have card readers for PCs. Why not combine the
two, so that you have a portable hard disk that can read your flash-card, memory stick or whatever you use, without a computer.
It doesn't need any kind of interface - you just insert the card, and it automatically downloads everything on the card to a date/time stamped folder. Runs on AA batteries.
When you get home, you just plug it into your computer like a normal USB hard drive.
It measures 12x7x3 cm, so you can easily carry it in a backpack or in your camera bag. Allows you to take 20gb of storage with you instead of 256mb, and is cheaper than buying extra flash cards.
It comes with ROM software for controlling I/O with storage media.
Optionally, it could even have a USB port which you could plug other USB drives into.
I don't think this would be difficult or expensive to make - card readers cost, what, US$20, USB hard disks US$100 ? I bet you could make one for less than US $150.