http://www.tagsistant.net/index.php
What is a semantic file system? Honestly, I don't know! Many people say that is something you can use to organize your contents using ontologies based on labels or tags. Others say it is related to relationship between files or "things". Others claim it has something to do with circles in fields! ;-)
To me, a semantic filesystem is basically a tool that allows one to catalogue files and to extract subsets using logical queries.
But why a filesystem and not a database or a desktop application? That's the most obvious but often hidden consideration a programmer should do! Software should be as interface neutral as possible. Interface neutral means that you should be able to communicate with this software easily and efficiently.
A desktop tool can be comfortable for the user, can be assistive and accessible. But other softwares will not be advantaged in interfacing with that tool. Perhaps other tools should be modified to apply to new API, requiring days of coding and possibly introducing new bugs! Universal, well known and well tested interfaces should be preferred against special, new, particular interfaces.
A filesystem is probably one (or the most) universal interface you can think of (especially on UNIX, and especially talking about files!). Later, someone will write a colorful and intuitive desktop application (or a series of) which will be appealing, but right now, with a 3 days hack, everyone can tag his/her files by simply using the filemanager interface he's used to.