Each new backup is being created with the same date-based filename - in my case "*-backup-2013-06-07.zip". Here is an extract from the log showing the actual date:
'01:21:09: Backup started on Friday July 12, 2013.
01:21:09: Your time limit is 30 seconds and your memory limit is 64M
01:25:33: Creating SQL backup of your WordPress core.
01:25:34: Creating SQL backup of your WordPress plugins.
01:25:53: Approximately 2% complete.
01:26:57: Approximately 57% complete.
01:27:00: Zip archive complete
01:27:00: Uploading *-backup-2013-06-07.zip (48.7MB) in chunks.'