Using Maria DB 5.2.7 + Percona Xtrabackup 1.6.2-274 I was unable to backup data (see below).

Errors:
root@ubuntu /tmp > /usr/bin/innobackupex --defaults-file='/etc/mysql/my.cnf' --password='*****************' --user='root' --host='localhost' --port='3306' --socket='/var/run/mysqld/mysqld.sock' /backup/db_full

InnoDB Backup Utility v1.5.1-xtrabackup; Copyright 2003, 2009 Innobase Oy
and Percona Inc 2009-2011. All Rights Reserved.

This software is published under
the GNU GENERAL PUBLIC LICENSE Version 2, June 1991.

110729 10:45:41 innobackupex: Starting mysql with options: --defaults-file='/etc/mysql/my.cnf' --password='******************' --user='root' --host='localhost' --port='3306' --socket='/var/run/mysqld/mysqld.sock' --unbuffered –
110729 10:45:41 innobackupex: Connected to database with mysql child process (pid=7070)
110729 10:45:47 innobackupex: Connection to database server closed
IMPORTANT: Please check that the backup run completes successfully.
At the end of a successful backup run innobackupex
prints "completed OK!".

innobackupex: Using mysql Ver 14.16 Distrib 5.2.7-MariaDB, for debian-linux-gnu (x86_64) using readline 5.2
innobackupex: Using mysql server version This software comes with ABSOLUTELY NO WARRANTY. This is free software,

innobackupex:: Warning: Ignored unrecognized line 2 in options : 'xtrabackup: Error: Please set parameter 'datadir'
'
innobackupex: fatal error: no 'mysqld' group in MySQL options
innobackupex: fatal error: OR no 'datadir' option in group 'mysqld' in MySQL options

I was able to fix the problem thanks to the suggestion of Wlad on Freenode/#mariadb.

The fix is as follows:

In /etc/mysql/my.cnf change "[server]" to "[mysqld]"

Wlad also indicated this issue caused other software to not work such as sql workbench.

Please consider changing this in the future to avoid compatibility issues with other software that expects "[mysqld]".