To all people who type "sudo su - root": the much simpler "sudo -i" accomplishes the same (-i launches an interactive shell).
(And both -i and -s can also not give you an interactive shell, if you give it a command to run. The point is that it will run the command in the shell rather than just fork()ing and exec()ing like a bare sudo invocation would do.)
$ sudo -s
$ pwd
/home/ralph
$ exit
$ sudo -i
root@orac:~# pwd
/root
root@orac:~# logout
$
$ sudo strace -fe execve sudo -s
execve("/usr/bin/sudo", ["sudo", "-s"], [/* 16 vars */]) = 0
execve("/bin/bash", ["/bin/bash"], [/* 16 vars */]) = 0
...
$ exit
$ sudo strace -fe execve sudo -i
execve("/usr/bin/sudo", ["sudo", "-i"], [/* 16 vars */]) = 0
execve("/bin/bash", ["-bash"], [/* 16 vars */]) = 0
...
root@orac:~# logout
$