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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.