I suppose you could put the PST file in the roaming profile directory
and then when they log in and out, the PST file would be stored on a
server.

Not sure if that gets your goals or not.

We store our PST files on a centralized server. It is a little
slower, but not too bad. If you have old hardware or use Netware, you
will see a much larger slow down.

We have a few users with their PST files in their roaming profile
directory. When these get around 500MB they can take a while to log
in and out.