iphoto external editing
tuffluffjimmy is right - I've been messing with iPhoto libraries as digital asset managers for a long time, and it can be done, but takes a fair bit of massaging.
iPhoto will save files that have been modified or edited in any way within it's own internal file structure and won't sync those up with external files in other folders.
If iPhoto prefs are set to NOT import files into the library, one ends up with two sets of folders, each with some of the images, but neither one complete.
This is - believe it or not - the way I prefer it. From Photoshop, I duplicate open files, close the originals, and re-save the edited versions to an outside folder, and when I'm done with a bunch of editing, I'll sync the two up by re-importing into iPhoto events, and merging the events back into one folder.
The posters who mentioned not deleting or moving files inside the iPhoto library file are dead right - it can result in very unpleasant things. However, one can copy an edited image FROM inside an iPhoto library file to an outside folder without damaging the iPhoto library file. Control-click iPhoto library file > Show Package Contents > Modified > Year of File Creation (most likely 2008), then navigate to the file, then Option-click and drag it out of there to copy to the Desktop or wherever.
Multiple iPhoto library files are a fantastic way to go, since iPhoto becomes insane to navigate when one gets up into dozens of events, or 500+ images (that's just my opinion).
Just hold down the Option and Command keys when launching Photo, wait a bit, and check off the boxes for what you want done (I usually just check all of them, and go have a sandwich).
I use iPhoto Library Manager (shareware) to sync up particular albums from one Library to another.
I use Chronosync to make incremental backups of these libraries from one drive to another (the Finder will back up the files, too - but it will do so by replacing the entire library file in the target drive - not much fun if you have a 1GB+ library file.
Another good move is to occasionally force iPhoto to rebuild its database, thumbnails, etc.
I've got over 10,000 images stored in about twenty different iPhoto libraries for different web clients. I've reached the point where continuing with iPhoto and this cumbersome system is too much, and unfortunately I just can't get along with Adobe's Bridge - so, I'm migrating everything over to Lightroom and will give that a try. Aperture was the other choice, and for a number of reasons I decided to go all-Adobe.
Happy hunting.