Open Image Capture, go to Preferences, and select something other than iPhoto for "When a Camera is connected, open:".
This is as stupid as putting the default browser setting in Safari's Preferences, but this been discussed before so there's no point in getting back into it.
BTW the topic name is misleading, as this is nothing to do with the "Open With..." menu you get when right/control-clicking on a document in the Finder.