Some users have reported an issue in which iPhoto 7.1.5 unexpectedly quits soon after launch or freezes indefinitey, necessitating a force-quit.
One MacFixIt reader writes:
"When I go to open iPhoto now it opens and then about five seconds later the cursor turns into a spinning ball and I am forced to force quit. [...] Now I am left with not being able to open my photos at all."
Problematic media In some cases, this issue is caused by problematic video, images, etc. that are stored in the individual user's media folders:
It now appears that the stalls may, in some cases, not be indefinite, but rather abnormally long and simply caused by the presence of extensive media libraries that must go through a similarly extensive scanning process.
When you see the spinning pinwheel, it's an indication that the main thread has been busy and is unresponsive for more than 5 seconds. It doesn't necessarily indicate an interminable hang, but is sometimes a precursor to one.
To check if an application has stalled permanently, without a good chance of recovery, check Activity Monitor (located in /Applications/Utilities as described here. There are some tell-tale signs, like extremely high processor usage (sometimes above 100%). You can also click the "Inspect" button after selecting the stalled application and check the "Recent hangs" total.
Rebuild database In some cases, this issue can be resolved by rebuilding the iPhoto database. This can be accomplished by launching iPhoto while holding down either the control or Command key. A menu will pop up and ask if you want to rebuild the photo files database. You should then check all the boxes and allow iPhoto to rebuild the database.
Delete iPhoto plist Finally, try deleting the file com.apple.iPhoto.plist then relaunching iPhoto.