I was having this same problem recently ie. iPhoto appeared to be in a loop (also 'non-responsive' in red type in the 'Force quit' dialogue under the Apple menu) at the end of supposedly importing large numbers of photos. I spent about 3 days trying to work out what was going wrong or whether there was a decent solution (read; wasted my time and generated stress). I'm posting this now in September 2012 because it might help someone, even Apple customer services, who are experiencing similar situations.
Also, word of warning, some people have suggested going into the iPhoto folder structure and moving stuff around in there etc. etc. It's not a good idea, really. You are basically playing with a very complex database and, most likely, you won't remember exactly what you did in there in order that you could undo it if something goes really wrong.
But I did dig into the iPhoto folders while that spinning icon was rotating endlessly while the iPhoto dialogue merely informs the user that it's 'finishing import'. What I discovered was that, when this is happening, iPhoto is actually still adding image files into the database, so, whatever you think, it's not in a loop at all. The problem is that when you're importing a huge amount of images at one time, like I was (approx. 50 000), that this 'finishing' process can take a long, long time...something like 20 hours for me. Well, after a couple of days I did manage to get my photos to open normally in iPhoto but it took a lot of patience.
So Apple, why couldn't you acknowledge this problem (which has been around it seems for around 3 years at least) and add a progress bar with 'remaining processing time' prediction that would appear during the 'finishing' process. This would surely also please iPhoto customers who really think that iPhoto is incapable of importing their images without crashing/looping and therefore giving up altogether/switching to other products.
Finally, now that all those images of mine are in the database iPhoto is insistant on analysing them for the face recognition function (which I don't use, don't want to use in future etc.). But it seems that you cannot turn off this function so I'm expecting that my Mac will now be eagerly processing all my images for faces for weeks or months to come, consuming processor time that I need for other things and burning unecessary electricity. (Actually it seems to really crash iPhoto at some point, if I leave iPhoto running overnight, for example).
Anyways, hope this post is useful to someone.
