I was wondering about that too, so I'm glad you posted your finding. So then I tried it using a CF card adapter in the PCMCIA slot of my TiBook. And it worked just as well. This is very useful because it means I don't have to drain my camera batteries to use Rendezvous picture sharing.
Also, the same trick works with the card readers that work with the USB Mass Storage devices. Put a camera-formatted memory card in your Mac. Add a couple images from your hard drive to the card's picture folder. Eject the card. Reinsert it and turn on Image Capture sharing. The new images will appear in your Rendezvous browser.
I would say that this is very cool.