I think the ability to exclude a sub folder of a shared folder is an important missing feature; http://www.ps3mediaserver.org/forum/viewtopic.php?f=6&t=14885.
For example say I want to share
but within that folder I want to exclude
User/Helen/Movies/Office party video
Perhaps one way to implement this might be by making PMS ignore folders that contain a text file in them called "noPMS.txt" ?