Interesting question.
Exchange 2000 does have a "deleted item retention" setting that can be enabled. Basically, it tells Exchange to hold on to any messages that have been deleted from the deleted items folder for "X" amount of days. You configure this on the mailbox store properties sheet, under the Limits Tab. Theres a setting for "keep deleted items" and "keep deleted mailboxes".
So the server provides you with the functionality you are looking for.
Now comes the sticky part, actually recovering the items. This is dead easy to do with Outlook. You just highlight the deleted items folder, select the "Tools" menu and then "recover deleted items". A window will appear with a list of deleted items that are still under the retention mechanism.
Since you use POP3, I assume your clients are a bit thinner than Outlook (Outlook Express maybe?). I'm not entirely sure you can do this with anything other than Outlook.
If you really needed to recover something, you could always attach an Outlook client to the mailbox in question, I suppose.