As enzotib said, Ubuntu has a root account but it's locked by default.

Now, about Macs:

As you can probably guess, root (along with all the daemon accounts) doesn't appear in the "Users & Groups" section of Settings.

Looking at my mac's `/etc/passwd`, there is an entry for `root`, along with a message 

    Note that this file is consulted directly only when the system is running
    in single-user mode.  At other times this information is provided by 
    Open Directory.

I tried to find the Open Directory user list, without success, but in the configurations there were mentions of explicitly giving root permissions even though they're implied.  I was never prompted for a root password when I first set the machine up, so I'd guess the root account is locked as it is in ubuntu.  I didn't (and don't really want to) try giving root a password and logging in with it, but you probably could.

The passwd line:

    root:*:0:0:System Administrator:/var/root:/bin/sh
The perl command from keith's comment adds a `:0` to the end.  No password hash.  There's no shadow file I can find either, I haven't been able to find them anywhere to check whether root might have a password.