Mac OS X and Windows Nitpick
This time, the same nit affects both OS's. This is a vendor-neutral nit.
Windows has UNC paths. UNC paths let you refer to a file using a simple syntax that includes the server, share name, and path within that share to a file you want. For example:
\\myserver\photos\2007\bob.jpg
This refers to a file named "bob.jpg" in a folder named "2007" on a share named "photos" on a server named "myserver".
Windows can cache my credentials for this server, so when I use Explorer to browse to this file, I don't have to manually log in; Windows does it for me.
On the Mac, there's a similar syntax. In the Finder, I can ask it open "smb://homserver/photos/2007" and that will show me the folder containing bob.jpg. Windows supports UNC paths at the file system level, but on the Mac, everything gets mounted into the /Volumes folder, so the smb:// basically says who does the mounting, and the rest of the path is handed off to Samba to mount.
Here's the problem: On both systems, the user-level file browser thing (Explorer / Finder) knows how to automatically authenticate and connect me to the server that has the file I want, but other applications don't get this automatic treatment. If a program that tries to directly open \\myserver\photos\2007\bob.jpg using CreateFile (Windows) or /Volumes/photos/2007/bob.jpg (Mac), both will fail.
This breaks the illusion that these folders are really just extensions of your desktop, and means the user has to remember to connect with the server before running your application.