rm -rf /

… does not work. At least not today on a Debian/Sid system. Looks like there are some things, not even root is allowed to do:

root@sid:~# rm -rf /
rm: cannot remove root directory ‘/'

If you’re curious what happens when you wipe your disk on a running system:

root@sid:~# cd /
root@sid:/# rm -rf *`

does the trick.

Update: That’s with plain rm not safe-rm.

Update 2: I looked through the code of rm.c to find that / is protected by default – as a simple rm --help also shows:

$ rm --help Usage: rm [OPTION]... FILE...
Remove (unlink) the FILE(s).

[...]

--no-preserve-root do not treat `/' specially
--preserve-root do not remove `/' (default)