Step 1: Launch Image Capture (Applications>Image Capture). If your iPhone isn't plugged in already, plug it in to the USB port on your Mac. It should then show up in the devices source list.
Step 2: Make sure you have the iPhone selected in the source list. When you do, you should see any photos you have on the iPhone appear in the right-hand column of the Image Capture application.
Step 3: At the bottom of the source list, you'll see your iPhone's name then, below that, you'll see the words "Connecting this iPhone opens:" and a drop-down menu. Select "No application." Now close the Image Capture app and you're done. No more iPhone opening up iPhoto, but your other cameras will still auto-launch iPhoto when they are connected!