I was just looking through a Japan Woodworker catalog and saw these pocket stones by EzeLap.
EzeLap Pocket Stones
I wouldn't use them for my own knives, but when I visit someone else and they have sh**y knives then they might come in handy. I often travel with a sharpened Forschner just in case I am called on for cooking duty, but this pocket-sized option might be better.
Has anyone tried these before? They only go up to 1200, but that would be fine for most normal cutlery to get reasonably sharp.