here is the log:
[INFO] BackWPup version 3.0.3; WordPress version 3.5.1; A project of Inpsyde GmbH developed by Daniel Hüsken
[INFO] This program comes with ABSOLUTELY NO WARRANTY. This is free software, and you are welcome to redistribute it under certain conditions.
[INFO] BackWPup job: faroeborn.com; DBCHECK+DBDUMP+DBOPTIMIZE+FILE+WPEXP+WPPLUGIN
[INFO] BackWPup cron: 0 3 * * 1; Mon, 11 Mar 2013 @ 03:00
[INFO] BackWPup job started from wp-cron
[INFO] PHP ver.: 5.3.3-7+squeeze14; apache2handler; Linux
[INFO] Maximum script execution time is 300 seconds
[INFO] MySQL ver.: 5.1.63-0+squeeze1
[INFO] curl ver.: 7.21.0; OpenSSL/0.9.8o
[INFO] Temp folder is: [snip]/wp-content/uploads/backwpup-98372-temp/
[INFO] Logfile folder is: /var/www/faroeborn.com/www/wp-content/backwpup-15538-logs/
[INFO] Backup type is: archive
[INFO] Backup file is: [snip]/wp-content/uploads/backwpup-[snip]-temp/[snip].tar.gz
[11-Mar-2013 03:00:18] Added XML export "faroe-born.wordpress.2013-03-11.xml.gz" with 71.78 kB to backup file list.
That's it... there is a backup in my S3 account as there should be which appears to be the right size, but the above log is sent in an e-mail to me, as expected, with "[SUCCESSFUL]" in the subject.
Triggering the job manually produces the exact same result.