- Locate the icon of your camera on the left pane of iPhoto. To the right of the camera icon is an eject button.
- Click on the eject button
If you have unmounted your camera and then want it to mount again, just unplug it from your Mac and plug it back in again.
I've noticed that at least in iPhoto '11, the iPhone does not have an unmount button and you can disconnect it without causing any problems.