Not a direct answer to your topic, but you can use the Image Capture program (instead of iPhoto) to quickly get at photos on the camera.
When I'm selling something, I'll use Image Capture to quickly move the pictures a temp folder on my desktop, bypassing iPhoto completely.
Googling around to find more info on the topic, a few people said that Canon didn't put the functionality in several models for it to act as a USB "mass storage" mode. 😕