Apple’s iPod Camera Connector: Supported Devices support article has an interesting tidbit about how to find out what USB protocol Mac OS X is using with your camera or card reader or similar device. The protocols they list for the iPod Camera Connector are PTP, Type 4, and Mass Storage; these correspond to protocols Mac OS X itself supports.
While the device is connected (and in my experience, has a card inserted or other storage available), open Image Capture. Click “Options” and then choose the “Information” tab. In Tiger, the “Device Module” line will tell you which protocol module is being used with the camera or card reader. It seems as if this is a useful tidbit of information, especially if troubleshooting is required.
By the way, I had to make sure that iPhoto wasn’t launched when I checked my SanDisk reader — otherwise, iPhoto tended to exert control over my card reader and it disappeared from Image Capture’s list. I assume the same could be true of Aperture or other applications which use the Image Capture framework.