Microsoft have now officially made available a Outlook backup tool, that will allow you to backup the PST files at regular intervals.
The Personal Folders Backup tool is a Outlook Add-in for Outlook 2007/2003/2002, that creates backup copies of your .PST files at regular intervals, in Outlook 2002 and later versions, making it easy to keep all of your Outlook folders safely backed up.
With Personal Folders Backup, you can choose which of your .PST files you wish to back up, and how often you wish to back them up. Each .PST file contains all of your Outlook folders, including the Inbox, Calendar, and Contacts.
Once you have downloaded and installed the add-in, restart Outlook, Open File –> Backup, to launch the Personal Folders Backup tool.
You can also choose how often a backup is done and which folders are included in the backup by choosing the Options button in the main backup window, if you want to store the backups in a different location or folder you can do that in the options window.
Download: Personal Folders Backup for Outlook [160Kb]