Here are two ways to do
1) Change the setting in iPhoto
Go to iPhoto's preferences and click on the "General tab", and then choose "No Application" in the pull down manual of "Connecting camera opens".
But after that, whenever you connect any digital camera to you Mac, you need to launch iPhoto manually.
2) Keep you iPhone Camera Roll album empty
After the import of your photos from iPhone to iPhoto is completed, tell iPhoto to delete the photos from your camera. This will remove all the photos and movies from your iPhone's Camera Roll. Then iPhoto will not automatically launch unless you have new media needs to be imported.
But if you need to keep a copy of those photos on your iPhone, then you need to place them into an album in iPhoto and then iTunes to sync those album with your iPhone.
Your call !
Information comes from http://www.maclife.com/article/howtos/kill_iphoto_autolaunch