I used to share the same iPhoto Library with my wife, using a standard unix symbolic link so that the folder "iPhoto Library" in my wife's home folder was actually a symbolic link pointing to my real folder in my home, and regularly change access rights to all files in the folder to allow my wife to work on it.
This hack is broken with iPhoto4, as it resets owner and access rights of the AlbumData.xml file. I found a simple solution to fix this issue, provided you have a volume onto which you disabled access rights :
% cd Pictures % ln -s /Volumes/YOUR_VOLUME_HERE/iPhoto Library
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040121141419759