Yes you can, in certain case.
I have about 1000 PC as TSM Client. Backup run during the day (between 08:00
AM and 08:00 PM). We have a specific management rule for PST (Only once a
week). We are able to backup PST File (On Win9x, NT, 2K Client and with
Office9x, 2000, XP). It doesn't works each time you try to backup. But
globaly it works. You will not have the last version  but one. You don't
loose every thing. You have to write it down on your SLA (Service Level
Agreement) with your client.

Regards,
Eric