Large scale mail migrations can be tricky. There is a shareware app that can be used to migrate pst files from the pst format into mbox, which can then be used with Mac OS X http://www.littlemachines.com.
If the migration process needs to be automated (they all seem to at scale) then a script could be written to crawl users, finds the pst files and then convert them. Or it could be done on the client side using a self-destructing launchd item. Conversion syntax for libpst would be something like the following:
readpst -o /output/folder /server/path/user.pst
Before you can use readpst, it needs to be built via libpst on the system that will run any scripts. Download libpst from http://alioth.debian.org/frs/?group_id=30390. This can be done with curl:
curl http://alioth.debian.org/frs/download.php/2492/libpst-0.5.3.tar.gz --O libpst-0.5.3.tar.gz
Next, extract the tar:
tar -zxvf libpst-0.5.3.tar.gz
Then cd into the new directory:
Then make libpst:
And now readpst should be available to convert mailboxes. This could be run from a centralized server or distributed to clients.