Hi Phil and welcome to our forum!
How about adding an e-mail submit action after the SharePoint submit that sends the form to the public folder? That's easier than creating a SharePoint event handler. There might be an easier way to do it with SharePoint 2007, but for 2003 I'd just have the e-mail submit as part of the submit. You can chain the actions to first submit to SharePoint and the send e-mail (assuming the SharePoint submit was successful). You'll have to either hardcode the public folder e-mail address, or use some dynamic rules in the form (in case you have multiple ones) or you could just do the distribution as part of the workflow on the Exchange server.
Good luck!
Patrick Halstead
Project Manager at Qdabra