Hardlinking directories isn't permitted. You get:
[root@here] ln --directory DIR1 DIR2
ln: cannot create hard link `DIR2' to `DIR1': Operation not permitted
Even though I'm root.
ln --help suggests that it should be possible:
-d, -F, --directory hard link directories (super-user only)
I tried is on RH 5.1 and 6.1+rawhide:
hardlinking directories is not possible with every filesystem (most filesystems
don't support this actually).
The ext2 filesystem definitely doesn't do hardlinked directories.
What do you need them for?