Microsoft doesn't recommend storing PST files on shared folders. It works, but you can end up making good friends with the SCANPST utility if you have dodgy network connectivity or the server computer is started while clients are "connected" to their PST files.
You cannot connect multiple Microsoft Outlook clients to the same PST file simultaneously. There was a very hackish replication function called "Net Folders" in some versions of Outlook. I never used it in production and found that it worked very poorly in test lab scenarios.
The "official" Microsoft solution for what you're looking for would be Exchange Server. That's a pretty expensive pill to swallow, but it'll do what you want.
Outlook 2003 doesn't "do IMAP" very well. It treats an IMAP server a lot like a POP3 server-- downloading data from the server and storing it locally. Outlook 2007 does a better job and preserves most of the "IMAP experience" of keeping data stored server-side. Having said that, though, I've never tried to "connect" multiple Outlook 2007 clients to the same IMAP folder at the same time. It should work fine, but you might see "strangeness" if multiple clients are interacting with the same messages simultaneously (filing them into subfolders at the same time, for example).
If you can't use Outlook 2007 you might want to consider using either Outlook Express or a free/open-source IMAP client (Thunderbird, for example) to get a better IMAP experience. Outlook 2003, though, is right out. Likewise attempting to "connect" multiple users to the same PST file simultaneously.