Searching through the previous threads on Geekzone they all seem to relate to syncing folders through some cloud based intermediary like Dropbox or SugarSync etc.
The only app I could find that syncs directly over the lan is Cheetah Sync. It seems to work ok - my only issue is that the sync can only be initiated from the phone, not the pc.
In my ideal dream world Kies Air would have folder sync functionality so you could drive everything from the pc.
What are you using?