Having a 128GB Air, it would be nice if the client could do selective sync but also sync to more than one volume. I keep my iTunes stuff on a separate drive, but that's one of the first things I would want to sync with 1TB of space.
I haven't tried this in a while, but I used to create symbolic links in my Dropbox to move things around the filesystem and it worked fine.
as @kalleboo suggested, symlinks are good too. I actually symlink my 'Google Drive' folder inside my Dropbox folder so I have that content "backed up', so to speak.