We find that (at least on Win2K) Outlook will release the .pst files after
some period of non-use.  But so far I haven't figured out exactly what that
period is, or any way to control it - (there was a .ini file that you could
set on earlier releases).  Any EXCHANGE experts out there who know more
about how this works?

But yes, the only way to guarantee availability of the .pst files for backup
is to close Outlook.