When I plug my Canon PowerShot digital camera into Snow Leopard, it automatically opens iPhoto and asks to import the pictures, but does not mount the camera on the desktop (from what I read, it's a Canon feature to prevent people from screwing up the file system on the memory card, or something). Is there a way I can move photos I've already imported back on to the camera? I can't seem to figure out how to move stuff from Snow Leopard to the camera using iPhoto, and since it doesn't mount....
The simplest way is to attach just the card with a USB card-reader. The card's file system mounts on your Mac and you can open it and drag files onto and off of it in the usual, familiar ways (or use Terminal's command line, if that's your bent).
Edit: I just tried connecting / talking to the camera. Only iPhoto and Image Capture seem be aware of it's even there; its file system is not mounted in OS-X. Both apps apparently can only read and delete photos. I couldn't find a way to write photos back to it or to see any other part of its file system. An upside of using a card reader is that it transfers photos quicker than the camera (at least than my old A70.)
I just noticed this question about the camera recognizing photos copied onto it. It might be relevant to what you're trying to do (though nothing more about how to get them there...)