Yes. That is, in fact, how Time Machine backs up so quickly. But I believe there are no userspace utilities to create them (and major POSIX assumptions break down, so you need to be careful if you do it in your everyday data).

Much to my surprise, rsync on osx (with the --link-dest arg, iirc) can create directory hardlinks. I was unable to figure out any other tool to delete them; I ended up having to rsync an empty directory on top.