It turns out that there is a setting which induces this behavior. It's tucked away into a small application called Image Capture.app. This application is ostensibly some sort of image importing application, but i've never seen it before. Among other things, it allows you to change the action to be performed when a "camera" device is plugged in. The setting is per-device, and you can opt to use any application (or no application). In theory you can even write your own complicated program to decide when to import, but for me the options iPhoto.app and "No Application" are sufficient.
(Note: this is on Snow Leopard. If you don't have Snow Leopard, YMMV)