Microsoft publishes a tool called pfbackup.exe. This utility will extract a snapshot of your PST files suitable for backing up by a tool such as bacula. Note that this will only work for local PST files, not for an Exchange server. The utility should be downloadable from

http://www.microsoft.com/downloads/details.aspx?FamilyId=8B081F3A-B7D0-4B16-B8AF-5A6322F4FD01&displaylang=en

Documentation for the utility is at

http://office.microsoft.com/en-us/assistance/HA010875321033.aspx