Here is how to safely unmount your camera:
- 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
When the camera icon disappears, you can safely disconnect your camera.
Click to see a larger image
If you just unplug your camera without unmounting, you will get the nasty message as shown in the second image. Worse than getting the nasty message, you can corrupt the data card on your camera so you can't access the photos on the card.
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.